Organometallic and Coordination Chemistry of the Actinides [1 ed.] 978-3-540-77836-3, 978-3-540-77837-0

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127 Structure and Bonding Series Editor: D. M. P. Mingos

Editorial Board: P. Day · X. Duan · L. H. Gade · T. J. Meyer G. Parkin · J.-P. Sauvage

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Organometallic and Coordination Chemistry of the Actinides Volume Editor: Thomas E. Albrecht-Schmitt

With contributions by S. C. Bart · F. G. N. Cloke · M. S. Eisen · K. Meyer M. Sharma · O. T. Summerscales

123

The series Structure and Bonding publishes critical reviews on topics of research concerned with chemical structure and bonding. The scope of the series spans the entire Periodic Table. It focuses attention on new and developing areas of modern structural and theoretical chemistry such as nanostructures, molecular electronics, designed molecular solids, surfaces, metal clusters and supramolecular structures. Physical and spectroscopic techniques used to determine, examine and model structures fall within the purview of Structure and Bonding to the extent that the focus is on the scientific results obtained and not on specialist information concerning the techniques themselves. Issues associated with the development of bonding models and generalizations that illuminate the reactivity pathways and rates of chemical processes are also relevant. As a rule, contributions are specially commissioned. The editors and publishers will, however, always be pleased to receive suggestions and supplementary information. Papers are accepted for Structure and Bonding in English. In references Structure and Bonding is abbeviated Struct Bond and is cited as a journal. Springer WWW home page: springer.com Visit the Struct Bond content at springerlink.com

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Preface

This volume reviews recent developments in the fields of organometallic and coordination chemistry of the actinides, and in particular uranium. Actinide chemistry in general has recently been rejuvenated with demonstrations of unprecedented structures, reactivity, and physical properties. While organouranium chemistry can be traced back to the Manhattan Project, most of these efforts were unsuccessful. However, by the mid-1950s the first uranium cyclopentadienyl (Cp) complexes were being reported, e.g. tricyclopentadienyl uranium(IV) chloride, (C5 H5 )3 UCl. The late 1960s heralded the synthesis and structural elucidation of “uranocene,” bis(cyclooctatetraenyl)uranium(IV), U(C8 H8 )2 , an expanded-ring sandwich compound that provided tantalizing evidence that 5f orbitals might be involved in bonding. One of the chapters in this volume details the expansion of this kind of work to include mixed sandwich U(III) cyclooctatetraene and pentalene complexes. As discussed by several of the authors, the availability of easily prepared mid-valent starting materials has been one of the primary factors involved in reinvigorating this field. Of particular interest to many readers will be the binding of small molecules by both organometallic and coordination compounds of uranium. Some of the holy grails of this chemistry include the activation of dinitrogen, carbon monoxide, and carbon dioxide. Various aspects of this work can be found in all three chapters, but are detailed in particular by O.T. Summerscales and F.G.N. Cloke. The origins of coordination compounds of uranium are difficult to define precisely because the definition of what constitutes a coordination compound versus a purely inorganic compound can be difficult to differentiate. However, the coordination chemistry of uranium is very old, dating back to at least the early 1800s. There is tremendous diversity in the type of ligands that have been found to form stable complexes with uranium. Recent work has focused on highly tailored ligand sets to yield specific physico-chemical responses. This work has included the development of uranium complexes that specifically bind small molecules such as carbon dioxide. In addition, heterometallic 3d-5f systems are now being developed to explore magnetic interactions. It is important not to overlook early pioneering efforts by T.J. Marks and co-workers, who among other key discoveries found that uranyl cations can template the formation of superphthalocyanines. S.C. Bart and K. Meyer’s chapter details

X

Preface

more recent advances in the coordination chemistry of uranium in mid- to high oxidation states. One of the most exciting and active areas of actinide research involves the development of novel catalysts. Thorium and uranium metallocene complexes have been shown to react in highly specific manners that in some cases parallel those of early transition metals, and in others the reactions are unique to the actinides. M. Sharma and M.S. Eisen’s chapter details metallocene organoactinide chemistry with a special focus on novel reaction pathways that have in some cases been deduced from thermochemical studies. In summary, the publication of this volume is a strong indicator of the substantial activity currently taking place in the organometallic and coordination chemistries of the actinides. The future promises to hold many more surprises. Auburn, December 2007

Thomas E. Albrecht-Schmitt

Contents

Metallocene Organoactinide Complexes M. Sharma · M. S. Eisen . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Activation of Small Molecules by U(III) Cyclooctatetraene and Pentalene Complexes O. T. Summerscales · F. G. N. Cloke . . . . . . . . . . . . . . . . . . . . .

87

Highlights in Uranium Coordination Chemistry S. C. Bart · K. Meyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Author Index Volumes 101–130 . . . . . . . . . . . . . . . . . . . . . . 177 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

Struct Bond (2008) 127: 1–85 DOI 10.1007/430_2008_082  Springer-Verlag Berlin Heidelberg Published online: 10 April 2008

Metallocene Organoactinide Complexes Manab Sharma · Moris S. Eisen (✉) Schulich Faculty of Chemistry, Institute of Catalysis Science and Technology, Technion – Israel Institute of Technology, Technion City, 32000 Haifa, Israel [email protected] 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

Synthesis and Reactivity of Actinide Complexes Trivalent Actinide Cp Complexes . . . . . . . . . Sterically Induced Reduction . . . . . . . . . . . Displacement of (C5 Me5 )1– Ligands . . . . . . . Bridging Complexes . . . . . . . . . . . . . . . . Affinity Towards Lewis Bases . . . . . . . . . . . Metal Ligand Back Donation . . . . . . . . . . . Beyond the Tris-Cp Complexes . . . . . . . . . .

. . . . . . . .

4 4 10 14 20 25 28 32

3

Tetravalent Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35

4

Conclusion and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . .

76

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract During the last decade, the chemistry of d0 /f n actinides has flourished reaching a high level of sophistication. Compared to the early or late transition-metal complexes and lanthanides, the actinides sometimes exhibit parallel but mostly complementary activities for similar organic processes, and sometimes even challenge the activities of the transition metals. A rapid increase in the numbers of reports in the Cambridge database also reflects their current importance. In view of the above, in this particular review we have provided an overview and updated information about the preparation and properties of the major classes of actinide complexes containing different cyclopentadienyl ligands and having the oxidation states (+3 and +4). Keywords Metallocene complexes · Organoactinides · Sterically induced reduction (SIR)

Abbreviations Cp Cyclopentadienyl, η5 -C5 H5 Cp′ η5 -C(CH3 )3 C5 H4  = Cp η5 -(CH3 )C5 H4 Cpφ η5 -{Si(CH3 )3 }C5 H4 Cp′′ η5 -1,3-{Si(CH3 )3 }2 C5 H3 # Cp η5 -1,3-{C(CH3 )3 }2 C5 H3 Cp∗ η5 -(CH3 )5 C5 Cp′′′ (CH3 )4 C5 COT η-C8 H8

2 THF DMSO tmeda pmdeta HMPA dddt bipy terpy py

M. Sharma · M.S. Eisen Tetrahydrofuran Dimethylsulfoxide Me2 NCH2 CH2 NMe2 (Me2 NCH2 CH2 )2 NMe OP(NMe2 )3 5,6-Dihydro-1,4-dithiin-2,3-dithiolate 2,2′ -Bipyridine 2,2′ :6′ ,2′′ -Terpyridine Pyridine

1 Introduction Organometallic chemistry has attracted much attention in recent years because of the structural novelty, reactivity, and catalytic applications. This interesting area of chemistry is building a bridge between organic and inorganic chemistry that involves a direct metal-to-carbon bond formation. With the advances of analytical techniques, researchers are able to investigate the chemistry to a much deeper level and, therefore, this subject is in the limelight of coordination chemistry. Since the preparation of ferrocene, the first metallic complex containing a π-bonding ligand, (η5 -C5 H5 )2 Fe [1], organometallic chemistry has traveled a long way from the early transition metals to the chemistry of electrophilic d0 /f n actinides complexes. In fact, the first well-characterized organoactinide complex, Cp3 UCl, was synthesized by Reynolds and Wilkinson [2] shortly after the synthesis of ferrocene. Actinides are relatively large in size, which facilitates high coordination numbers. The availability of 5f valance orbitals make them chemically and coordinatively different from the d-block elements. The interests of researchers have been stimulated by the effective employment of the 5f and 6d orbitals in chemical bonding. Again, in comparison to the lanthanides, actinides are much more prone to complex formation as the 5f orbitals have a greater spatial extension relative to the 7s and 7p orbitals than the 4f orbitals have relative to the 6s and 6p [3]. Unlike the early or late transition-metal complexes and lanthanides, the actinides exhibit parallel but mostly different reactivities for similar organic processes, sometimes challenging the activities of transition metals and shedding light on their unique reactivities. Most developments in the non-aqueous chemistry of the actinides have involved the use of thorium and uranium, both due to their lower specific activity, and to the apparent chemical similarity to group IV metals in organometallic transformations. At present, among these, the coordination chemistry of uranium is drawing extra attention, which is evident from the statistical data of crystal structures in the Cambridge structural database. The compounds of uranium and thorium that have been crystallographically characterized during 2003 to 2007

Metallocene Organoactinide Complexes

3

are almost 1.3 times more than those reported during the previous 5 years (Fig. 1), whereas all the molecular complexes of the 3d transition metals and f elements increased by a factor of around 1.5 times [4]:

Fig. 1 Number of thorium and uranium complexes crystallographically characterized from 1978 to 2007

The electronic states of the actinides are also interesting as the energies of the 5f , 6d, 7s and 7p orbitals are very close to each other over a range of atomic number (especially U to Am) [3, 5] and might be responsible for a broad spectrum of oxidation states. Uranium has further demonstrated the ability to access a wide range of oxidation states (III to VI) in organic solvents, providing for greater flexibility in affecting chemical transformations. During the 1960s, the main technological interest in organoactinide chemistry lay in its potential for application in isotope separation processes [5], but at present the advancement of sophistication has put impetus on the interest of actinide chemistry towards the stoichiometric and catalytic transformations, particularly in comparison to d-transition metal analogs. In many instances the regio- and chemo-selectivities displayed by organoactinides are complementary to those observed for other transition-metal complexes. The reactivity of organoactinide complexes lies in their ability to perform bondbreaking and bond-forming reactions of distinct functional groups. Steric and electronic factors play key roles in such processes. While discussing the steric effect, Xing-Fu suggested that the stability of a complex is governed by the sum of the ligand cone angles [6–8]. According to this model, highly coordinative “oversaturated” complexes will display low stability. An additional model for steric environments has been proposed by Pires de Matos [9]. This

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M. Sharma · M.S. Eisen

model assumes pure ionic bonding, and is based on cone angles defining the term “steric coordination number”. A more important and unique approach to the reactivity of organo-5f complexes regards the utilization of thermochemical studies. The knowledge of the metal–ligand bond enthalpies is of fundamental importance for the estimation of new reaction pathways [10–17]. In addition, neutral organoactinides have been shown to follow activation via a four-center transition state (Eq. 1) due to the high-energy orbital impediment to undergo oxidative addition and reductive elimination. Such a transition state allows the predictions of new actinide reactivities, when taking into account the negative entropies of activation [18]: (1) Several general review articles [19–41] dealing mostly with the synthesis of new actinide complexes confirm the broad and rapidly expanding scope of this field. Those reviews dealt with the structure, stability, and reactivates of complexes with cyclopentadienyl, dienyls (pentadienyl, cyclohexadienyl, indenyl, phospholyl), cyclooctatetraenyl, arene ligands, hydrocarbyls, and hydrides ligands. In this particular review we will provide an overview of the preparation and properties of the major classes of actinide complexes containing different cyclopentadienyl ligands. Discussions are classified on the basis of formal oxidation states and we are confining our discussions only to the oxidation states III and IV.

2 Synthesis and Reactivity of Actinide Complexes The rapid growth of the organoactinide chemistry is intimately coupled with the use of the π-coordinating cyclopentadienyl (Cp) or modified Cp ligands. Since the first report of the complex (Cp)3 UCl by Wilkinson [2], followed by Fischer’s report of cyclopentadienyl compound of U and Th [42], a plethora of Cp actinide complexes have been synthesized. The most interesting part of this chemistry is that it is possible to coordinate one, two, three, or four Cp ligands in an η5 -coordination mode [43–48] and that it has the ability to stabilize a wide variety of oxidation states and coordination environments [3]. 2.1 Trivalent Actinide Cp Complexes Among the bis-, tris- or tetrakis-Cp complexes, a overwhelming number of homoleptic tris-Cp (or modified Cp) complexes of the type (η5 -C5 H5 )3 An

Metallocene Organoactinide Complexes

5

(An = actinide) has been reported. During the period of 1960s to 1970s a spurt of activity concerning the primary investigation of trivalent organoactinides was observed, after that there was a dormant period till 1987 when Cramer, Gilje, and coworkers uncovered a rich chemistry in the reactions of Cp3 UCl with lithiated phosphoylides and related molecules [49, 50]. The reaction of UC13 , with Cp-, or the reduction of Cp3 UX in the presence of neutral Lewis bases (L), was shown to produce a variety of U(III) complexes Cp3 UL [50, 51]. Later, it has been reported that these ligands support most members of the actinide series from thorium to californium to form complexes of the type (η5 -C5 H5 )3 An (An = actinide). These complexes exhibit a wide variety of novel structures and reactivities, including uranium–carbon multiple bonding. A number of synthetic routes have been reported to generate these species and their tetrahydrofuran (THF) adducts, including direct metathesis with alkali metal salts [50, 52–55], or transmetallation with Be(η5 -C5 H5 )2 or Mg(η5 -C5 H5 )2 [56–62]. In addition, the trivalent compounds may be obtained from chemical [53] or photochemical [63, 64] reduction of suitable tetravalent actinide precursors [51, 65–67]. Examples of these preparations are given in Eqs. 2–5:

(2)

(3) (4)

(5) Le Marechal and coworkers [68] showed a new method for the preparation of Cp3 U(THF) by the treatment of Cp3 UCl with sodium amalgam, Na/Hg, in the presence of 18-crown-6-ether and NaH (Eq. 6):

(6)

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M. Sharma · M.S. Eisen

Table 1 NMR data of actinide complexes Compound

Chemical shifts in C6 D6 (ppm)

Refs.

(MeC5 H4 )3 U·THF

–11.62 (6H), –13.99 (4H), –14.39 (6H), –15.61 (9H), –31.06 (4H) –19.2 (2H), –18.7 (9H), 9.2 (2H) –21.0 (9H), –24.2 (2H), 9.04 (2H) 20.8 (s, 1H, CH–Cp′ ), –4.8 (s, 2H, CH–Cp′ ), –9.3 (s, 18H, SiMe3 –Cp′ ) 7.4, –35.5 (s, 6H each, CH3 –Cp′ ), –5.0 (s, 1H, CH–Cp′ ) –0.93

[72]

(MeSiC5 H4 )3 U (Me3 CC5 H4 )3 U [1,3-(Me3 Si)2 (C5 H3 )]3 U [(C5 Me4 H)3 U] (Me5 C5 )3 U

[72] [73] [75] [74] [85]

They also reported the successful synthesis of [Cp3 UCl][Na(18-crown-6)], and [(Cp3 U)2 (µ-H)][Na(THF)2 ] (Cp = η5 -C5 H5 ) by following the same procedure. The solubility of the parent tris(cyclopentadienyl)actinide, Cp3 An, complexes is limited in non-polar media, presumably due to oligomerization through bridging cyclopentadienyl ligands. Therefore, the synthesis of the most soluble iodine starting material, AnI3 L4 (An = U, Np, Pu; L = THF, pyridine, DMSO) [69], perhaps can be considered as the major useful development of the actinide coordination chemistry. These species, generated from actinide metals and halide sources in coordinating solvents, are readily soluble in organic solvents, and serve as convenient precursors to a variety of trivalent actinide species [70]. Later, the hurdle of solubility was attended by a number of researchers by synthesizing a variety of substituted-tris-Cp ligand complexes (Table 1). A wide spectrum of synthetic routes have been proposed for these precursors [71–75] (Eqs. 7–11):

(7)

(8)

Metallocene Organoactinide Complexes

7

(9)

(10)

(11) Until 1999 the only reported single crystalline compound of thorium(III)Cp was [ThCp′′ 3 ]{Cp′′ = η5 -C5 H3 (SiMe3 )2 -1,3}. In 1986, Blake and coworker [76] reported the synthesis of dark blue crystalline complex [ThCp′′ 3 ]

Fig. 2 Crystal structure of [η5 -(Me3 Si)2 C5 H3 ]3 Th [76]. Reproduced with permission from the Royal Society of Chemistry

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(Fig. 2) by the reduction of [Cp′′ 2 ThCl2 ] in toluene with sodium–potassium alloy along with metallic Th as the byproduct. Later a preferred route was reported [77] (Scheme 1) for the synthesis of this complex by using [Cp3 ThCl] as the precursor, which could control the production of Th byproducts.

Scheme 1 Synthetic routes of [Th{C5 H4 (R/R′ )}3 ]; R = SiMe3 ; R′ = SiMe2 Bu-t [76, 77]

Most of the base-free tris(cyclopentadienyl)thorium complexes crystallize in a pseudotrigonal planar structure, with averaged (ligand centroid)– Th–(ligand centroid) angles near 120◦ , and average Th–Cring distances of A. One of the most investigated aspects of actinide–cyclopentadienyl 2.80(2) ˚ chemistry is the nature of the bonding between the metal and the ligands [78]. Experimental studies of tris(cyclopentadienyl)actinide complexes, including 237 Np Mössbauer studies of (η5 -C H ) Np [51] and infrared and absorp5 5 3 tion spectroscopic studies of plutonium, americium, and curium analogs [57, 79, 80] suggest that the bonding is somewhat more covalent than that of lanthanide analogs, but the interaction between the metal and the cyclopentadienyl ring is still principally ionic. Theoretical treatments have suggested that the 6d orbitals are chiefly involved in interactions with ligand-based orbitals. While the 5f orbital energy drops across the series, creating an energy match with ligand-based orbitals, spatial overlap is poor, precluding strong metal–ligand bonding [44]. Thorium lies early in the actinide series and the relatively high energy of the 5f orbitals (before the increasing effective nuclear charge across the series drops the energy of these orbitals) has lead to speculation that a Th(III) compound could in fact demonstrate a 6d1 ground state. In support of this, Kot and coworkers [81] have reported the observation of an EPR spectrum with g values close to 2 at room temperature. Although in actinide and lanthanide chemistry the use of permethylated cyclopentadienyl (C5 Me5 – ) species as ligand is quite common, for a decade it was thought that the molecules of formula [M(C5 Me5 )3 ] will be sterically too crowded to exist as the cone angle of (C5 Me5 – ) was thought to be more larger than the 120◦ necessary for [M(C5 Me5 )3 ] complexes [82–84]. However, the synthesis of [U(C5 Me5 )3 ] by Evans et al. [85] opened the door to a new era

Metallocene Organoactinide Complexes

9

of cyclopentadienyl-actinide chemistry. The complex (η5 -C5 Me5 )3 U was prepared by the reaction of a trivalent hydride complex with tetramethylfulvene (Scheme 2) and, in fact, a direct metathesis route has not yet proven successful. The discovery of two or three electron reductivity of this complex had stimulated researchers to develop better synthetic routes.

Scheme 2 Multiple synthetic routes of [(Me5 C5 )3 U] [85, 86]

Evans [86] and coworker have developed a modified pathway for the synthesis of Cp∗ 3 U with improved yield up to 92% (Scheme 3). The molecular structure of the complex is shown in Fig. 3. The average U–Cring bond disA) is much larger than in other crystallotance in this compound (2.858(3) ˚ graphically characterized U(III) pentamethylcyclopentadienyl complexes (ca. A), suggesting a significant degree of steric crowding. 2.77 ˚

Scheme 3 Sterically crowded U(III) complexes [95, 96]

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M. Sharma · M.S. Eisen

Fig. 3 Crystal structure of (η5 -C5 Me5 )3 U [85]. Reproduced with permission

2.2 Sterically Induced Reduction The synthesis of Cp∗ 3 U{Cp∗ = η5 -C5 Me5 } carved a new path for the researchers to go one step ahead in the electrochemical studies of organoactinide complexes. The reduction reaction involving more than two electrons are not common for metal complexes containing just one metal. However, the synthesis of complexes of the type Cp∗ 3 M led to the development of “sterically induced reduction” (SIR) chemistry in which sterically crowded complexes of redox inactive metals act as reductants [87, 88]. Evans et al. showed that the sterically induced reduction couple, U(III)/U(IV), can act as a multielectron reductant [89]. As an example, Cp∗ 3 U reacts as a three-electron reductant with 1,3,5,7-C8 H8 , (Eq. 12) in which one electron arises from U(III) (Eq. 13) and two result from two C5 Me5 – /C5 Me5 half reactions (Eq. 14) presumably via SIR. This phenomenon was further corroborated by the stepwise reduction of phenyl halide with Cp∗ 3 U (Eq. 15): (12) (13) (14)

Metallocene Organoactinide Complexes

11

Fig. 4 Crystal structure of a (η5 -C5 Me5 )3 U(CO) [95] and b (η5 -C5 Me5 )3 U(N2 ) [96]. Reprinted with permission from [95, 96];  (2003) American Chemical Society

(15) Sterically hindered metal centers for the class of compounds of the type (η5 -C5 Me5 )3 M have unusually long metal ligand bonds. Although long metal ligand distances are known in f -element complexes containing agostic interactions [90], they generally involve only one or two interactions and the rest of the bonds are normal and predictable based on ionic radii [91–93]. In contrast, in the molecule (η5 -C5 Me5 )3 U all the metal–ligand bonds were longer than the conventional distance [94]. This phenomenon further induced researchers to investigate the reaction chemistry of the (η5 -C5 Me5 )3 U moiety. The reaction of (η5 -C5 Me5 )3 U with CO produced the sterically more crowded U(III) complex Cp∗ 3 U(CO) [95]. With N2 [96] it afforded the monometallic complex Cp∗ 3 U(η1 -N2 ) (Fig. 4), demonstrating that end-on nitrogen coordination is possible for f -elements (Scheme 3)(vide supra). In fact, this is the first monometallic f -element complex of N2 of any kind because it is most commonly found as M2 (µ-η2 :η2 -N2 ) moieties involving (N2 )–2 [97–103]. The binding of N2 in Cp∗ 3 U(η1 -N2 ) was found to be reversible, i.e., it releases N2 when the pressure was lowered to 1 atm in a solution of C6 D6 , quantitatively regenerating the parent complex (Scheme 3). In contrast, a solution of Cp∗ 3 U(CO) was found to be stable for hours under Ar or vacuum. The discovery of the SIR phenomenon for this kind of complex opened the window for the multi-electron reduction system. The complex Cp∗ 3 U shows a reductive coupling of acetylene [104] and reductive cleavage of azobenzene [105] (Scheme 4), two- and four-electron processes, respectively. These U(IV) and U(VI) complexes can also be obtained from [(η5 -C5 Me5 )2 U][(µ-Ph)2 BPh2 ], where [BPh4 ]– acts as a one-electron reductant (Scheme 4) [104]. On the other hand the reaction of (η5 -C5 Me5 )3 U and cyclooctatetraene giving [{U(η5 -C5 Me5 )(C8 H8 )}2 (µ-C8 H8 )] is accomplished through one U(III)/U(IV) and two (C5 Me5 )– /C5 Me5 couples [89].

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Scheme 4 Sterically induced reduction chemistry of [(Me5 C5 )3 U] [89, 104–107]

With KC8 in benzene it gave [{U(η5 -C5 Me5 )2 }2 (µ-η6 :η6 -C6 H6 )] [106], which itself proved to be also quite interesting in the area of multi-electron reduction chemistry, as shown by its reaction with azobenzene in which it functions as an eight-electron reductant [104]. Thus, it is clear that the complex (η5 -C5 Me5 )U shows quite an interesting two- to fourelectron reduction chemistry in a similar way to that of the com-

Scheme 5 Multielectron oxidation/reduction couple of SIR uranium system [104]

Metallocene Organoactinide Complexes

13

Fig. 5 Crystal structure of [(η5 -C5 Me5 )2 U]2 (µ-O) [107]. Reprinted with permission from Elsevier

plexes [(η5 -C5 Me5 )2 U][(µ-Ph)2 BPh2 ] and [{U(η5 -C5 Me5 )2 }2 (µ-η6 :η6 -C6 H6 )] (Scheme 5). Another most interesting and unusual result has been reported regarding the formation of an U(III) oxide complex, [(Cp∗ )2 U]2 (µ-O) (Fig. 5) obtained by the (η5 -C5 Me5 )3 U reduction system (Scheme 4) [107]. This has been claimed to be the first molecular trivalent uranium oxide so far reported. The complex was isolated from a reaction of (η5 -C5 Me5 )3 U with KC8 in toluene. Probably this is the first example of an SIR process in which the C5 Me5 –1 reduction precedes the U(III) electron transfer. Further investigating the SIR, Evans et al. [106] reported that [(Cp∗ )2 U]2 (µ-η6 :η6 -C6 H6 ) functions as a six-electron reductant in its reaction with three equivalents of cyclooctatetraene to form [(Cp∗ )(C8 H8 )U]2 (µ-η3 :η3 -C8 H8 ), (C5 Me5 )2 , and benzene (Eq. 16):

(16) This multi-electron transformation can be formally attributed to three different sources: two electrons from two U(III)/U(IV) reaction, two electrons from sterically induced reduction by two (C5 Me5 )1– /(C5 Me5 ) ligands, and two electrons from a bridging (C6 H6 )2– /(C6 H6 ) process. Apart from these SIR processes, there are few other examples where the U(III) center undergoes one- or two-electron oxidation. It was found that alkyl halides can also be oxidized by U(III) to generate equimolar mixtures of U(IV)–R and U(IV)–X as shown in Eq. 17 [108]: (17)

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There are few examples on the oxidation of the U(III) center to give corresponding U(IV) complexes. Stults et al. [73] showed that unlike the cerium metallocene, the uranium metallocene (MeC5 H4 )3 U(THF) behaves quite differently with alcohols and thiols. It forms the U(IV) complex of the type [(MeC5 H4 )3 UER], where ER is OMe, OCHMe2 , OPh, or SCHMe2 . The different type of reaction shown by cerium and uranium metallocenes towards alcohols and thiols most likely reflects the case of oxidizing uranium: the U(IV)/U(III) couple is – 0.63 V in aqueous acid whereas the Ce(IV)/Ce(III) is + 1.74 V [73]. Even the uranium complex with the more crowded substituent on the cyclopentadiene, i.e, (Me3 CC5 H4 )3 U, also undergoes a similar type of reaction with thiophenol (Eq. 18) to give (Me3 CC5 H4 )3 USPh:

(18) 2.3 Displacement of (C5 Me5 )1- Ligands Significant steric hindrance and the longer M–C bond in complexes of the type (C5 Me5 )3 M induced another type of reaction, i.e., (C5 Me5 )1– substitution reaction from the metal center [106]. Removal of (C5 Me5 )1– is very reasonable due to the steric crowding in these long bonded organometallics. The loss of (C5 Me5 )1– anions from (C5 Me5 )3 M complexes by η1 -alkyl or SIR pathways is well known [94], but the removal of (C5 Me5 )1– rings from f element complexes by ionic metathesis is not a common reaction. The synthesis of [(Cp∗ )2 U]2 [(µ-η6 :η6 -C6 H6 )] provides an example of the (C5 Me5 )1– displacement from the (Cp∗ )3 U moiety. Initially the complex [(Cp∗ )2 U]2 [(µ-η6 :η6 -C6 H6 )] was obtained as a byproduct in the synthesis of (Cp∗ )3 U from [(Cp∗ )2 U][(µ-Ph2 )BPh2 ] [86], but it could have been synthesized directly from (C5 Me5 )3 U [106] (Scheme 4) (vide supra). In an another synthetic route, it was also synthesized from [(Cp∗ )2 U][(µ-Ph2 )BPh2 ], the precursor to (C5 Me5 )3 U, in combination with K/18-crown-6/benzene as shown in Eq. 19:

(19)

Metallocene Organoactinide Complexes

15

The complex [(C5 Me5 )2 U]2 [(µ-η6 :η6 -C6 H6 )] was structurally characterized as a bimetallic species in which an arene ring is sandwiched between A, two [(C5 Me5 )2 U] moieties. The U–U distance was found to be 4.396 ˚ and the (C5 Me5 )1– rings arranged tetrahedrally around the U–C6 H6 –U core. In a similar manner, the amide analog, {[(Me3 Si)2 N](C5 Me5 )U}2 [(µ-η6 :η6 -C6 H6 )], was synthesized by displacement of two (C5 Me5 )1– moieties from [(C5 Me5 )2 U]2 [(µ-η6 :η6 -C6 H6 )] when it reacts with two equivalents of KN(SiMe3 )2 (Eq. 20). The comparison of the crystallographic data (Table 2) of these two complexes revealed that the average U–C(C5 Me5 ) bond distances in {[(Me3 Si)2 N](C5 Me5 )U}2 [(µ-η6 :η6 -C6 H6 )] is shorter than that of [(C5 Me5 )2 U]2 [(µ-η6 :η6 -C6 H6 )] as well as the angle (C5 Me5 ring centroid)–U –(C6 H6 ring centroid) in the former is smaller than the latter (Table 2). Thus, incorporation of the (Me3 Si)2 N ligand into the coordination sphere reduced the steric constrain around the metal centers. In {[(Me3 Si)2 N](C5 Me5 )U}2 [(µ-η6 :η6 -C6 H6 )] (Fig. 6), the two C5 Me5 ring centroid, the two N-donor sites took a square plane arrangement rather than the sterically more compact tetrahedral arrangement of the four rings as in [(C5 Me5 )2 U]2 [(µ-η6 :η6 -C6 H6 )] [106]:

(20)

Fig. 6 Crystal structure of [(Me3 Si)2 NCp∗ U]2 (µ-η6 :η6 -C6 H6 ) [106]. Reprinted with permission from [106];  (2004) American Chemical Society

Bz benzene a Centroid b Average

U(1)–U(2) U(1)–Cp1 a U(1)–Cp2 a U(1)–Bz a U(1)–Cp1 b U(1)–Cp2 b U(1)–Bz b Cp1 a –U(1)–Cp2 a Cp1 a –U(1)–Bz a Cp2 a –U(1)–Bz a

4.396 2.567 2.583 2.194 2.835 2.852 2.621

[(C5 Me5 )2 U]2 (µ-η6 :η6 -C6 H6 ) Parameters Bond distance (˚ A)

121.1 118.9 119.8

U(1)–U(2) U(1)–Cp1 a U(1)–Bza U(1)–N(1) U(1)–Cp1 b U(1)–Bz b Cp1 a –U(1)–Bz a Cp1 a –U(1)–N Bz a –U(1)–N

{[(Me3 Si)2 N](C5 Me5 )U}2 (µ-η6 :η6 -C6 H6 ) Bond angle Parameters (◦ ) 4.219 2.506 2.146 2.306 2.781 2.540

Bond distance ˚) (A

130.9 111.2 117.8

Bond angle (◦ )

Table 2 Comparison of bond length (˚ A) and bond angle (◦ ) of [(C5 Me5 )2 U]2 (µ-η6 :η6 -C6 H6 ) and {[(Me3 Si)2 N](C5 Me5 )U}2 (µ-η6 :η6 -C6 H6 )[106]

16 M. Sharma · M.S. Eisen

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17

While investigating the nature of bonding in these complexes, Evans et al. reported [106] an arene exchange reaction in which a bridged xylene complex was formed by the reaction of (C5 Me5 )3 U, KC8 and p-xylene as shown in Eq. 21:

(21) The synthesis of (C5 Me5 )2 U[N(SiMe3 )2 ] by the addition of KN(SiMe3 )2 to a solution of (C5 Me5 )3 U in C6 D6 provides an excellent example of an ionic metathesis reaction [106] (Scheme 6). (Cp∗ )2 U[N(SiMe3 )2 ] has been previously synthesized by addition of M[N(SiMe3 )2 ] (M = Na, K) to [(Cp∗ )2 UCl]3 [109, 110] or (Cp∗ )2 UMe2 K [86]. It can also be obtained from KN(SiMe3 )2 and [(Cp∗ )2 U][(µ-Ph2 )BPh2 ], a complex that is an excellent reagent for ionic metathesis reactions because it contains the [(Cp∗ )2 U]+ cation loosely ligated by bridging η2 -arenes of the (BPh4 )1– anion [106].

Scheme 6 Synthetic routes for [(C5 Me5 )2 U][N(SiMe3 )2 ] [86, 106, 109]

Manriquez et al. [109] showed an example of unusual chemistry of the uranium pentamethylcyclopentadienyl complex. It was found to form a tetrameric {U[η5 -(CH3 )5 C5 ]2 Cl}3 unit, which was synthesized by different

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procedures as shown in the following reactions (Eqs. 22–25):

(22)

(23) (24) (25) The structural analysis shows that the crystals composed of discrete trinuclear {[η5 -(CH3 )5 C5 ]U(µ2 -Cl)}3 molecules (Fig. 7) in which each U(III) ion adopts the familiar pseudotetrahedral “bent sandwich” configuration. The A) six-atom bridging Cl– ligands serve to generate the planar (to within 0.02 ˚ (–U–CI–)3 ring.

Fig. 7 Perspective drawing of the {U[η5 -(CH3 )5 C5 ]2 Cl}3 molecule [109]. Reprinted with permission from [109];  (1979) American Chemical Society

The complex {[η5 -(CH3 )5 C5 ]U(µ2 -Cl)}3 is insoluble in hydrocarbon solvents, but readily dissolves in the presence of Lewis base donors to form the corresponding adducts (Scheme 7) [109]. Once the solubility problem was solved, the chemistry of this molecule was investigated in various reactions, like alkylation with the sterically bulky lithium reagent, preparation of organouranium(III) amide, and reductive coupling of alkyne as shown in Scheme 7. Apart from these persubstituted Cp-ligand complexes, mono- and disubstituted Cp-ligand complexes are also in the race to achieve the milestone. The f -elements, when they form complexes with a variety of cyclopentadienyl ligands, possess an extensive organometallic chemistry that includes,

Metallocene Organoactinide Complexes

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Scheme 7 Reaction chemistry of {(η5 -Me5 C5 )2 U(µ2 -Cl)}3 [109]

among other interesting features, metal-to-carbon bonding, metal-to-X atom σ-bond formation, Lewis base adduct formation, ligand reductive coupling etc., all of them with unique features. The tris(cyclopentadienyl)actinide complexes sheds light on the nature of metal orbital participation in chemical bonding. Actinide metals generally are acidic and coordinate to Lewis bases. Therefore, to understand these unique features, the chemistry of these mono- or di-substituted cyclopentadienyl complexes have been studied extensively. As previously discussed, many of the tris(cyclopentadienyl)actinide complexes can be isolated as THF adducts directly from reactions carried out in that solvent. Trivalent uranium metallocenes form compounds of the type (RC5 H4 )3 U(L) where R is either H or CH3 and L is a Lewis base such as tetrahydrofuran [111], tetrahydrothiophene [112], 4-dimethylaminopyridine [113], trimethylphosphine [114], or 1,2-bis-(dimethylphosphino)ethane [115]. All of these molecules may be described as four-coordinate complexes of trivalent uranium (defining the midpoint of the cyclopentadienyl ring centroid as occupying one coordination position) with a distorted tetrahedral stereochemistry.

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2.4 Bridging Complexes During the 1980s, Zalkin and coworkers synthesized a variety of complexes to profoundly study the coordination chemistry of tetra- and trivalent uranium complexes with a variety of mono-, or bidentate ligands. A phosphorus bridge uranium complex [U(C5 H5 )3 ]2 {(CH3 )2 PCH2 CH2 P(CH3 )2 } was synthesized (Eq. 26) [115] in which the P–P ligand did not act as a bidentate chelating ligand but indeed formed a bridge in a monodetate fashion between two trivalent U units (Fig. 8):

(26) Therefore, the geometry of the complex is quite unusual with respect to the type of the ligand. The reason for this structural change is presumably the steric hindrance around the metal center. The coordination geometry around each uranium atom is quite similar to that in (CH3 C5 H4 )3 U{P(CH3 )3 } (vide infra) [114]. The average U–P bond distance in A, which is almost same [U(C5 H5 )3 ]2 {(CH3 )2 PCH2 CH2 P(CH3 )2 } is 3.022(2) ˚ as that found in (CH3 C5 H4 )3 U{P(CH3 )3 } (2.972(6) ˚ A). Another U(III) bridging complex, [Li(tmeda)2 ] · [Li(tmeda)]2 [µ-MeC5 H4 ] [(MeC5 H4 )U]2 [µ-Me], was obtained by the addition of one molar equivalent of methyllithium to (C5 H4 Me)3 U(THF) in diethyl ether in the presence of one molar equivalent of Me2 NCH2 CH2 NMe2 (tmeda) at – 30 ◦ C [116]. The crystal structure of the complex reveals that it contains one molecule each of Li(tmeda)2 and [Li(tmeda)]2 [µ-MeC5 H4 ], and two molecules of [(MeC5 H4 )3 U]2 [µ-Me]. The geometry of the anion has an U–C–U angle of

Fig. 8 Molecular structure of [U(C5 H5 )3 ]2 {(CH3 )2 PCH2 CH2 P(CH3 )2 } [115]. Reprinted with permission from IUCr Journals, http://journals.iucr.org

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21

A. The U–C (bridging) dis176.9◦ and U–C distances of 2.71(3) and 2.74(2) ˚ A and [Cp3 U(n-Bu)]– tance is long relative to Cp3 U(n-Bu) [117] with 2.43(2) ˚ ˚ with 2.56(1) A [118], as expected due to bridging. The bonding was explained with the help of the D3h symmetry methyl anion, which was formed from sand two p-orbitals giving a sp2 -hybridized set that contains six electrons for the C–H bonds and an unhybridized p-orbital with its two electrons that can be used in bonding with the σ-orbitals on the Lewis acid (MeC5 H4 )3 U. A series of halide bridge U(III) complexes of the type [Cp′′ 2 U(µ-X)]n (where Cp′′ = 1,3-(SiMe3 )2 C5 H3 ; X = F, Cl, Br or I) have been synthesized [119] by the reduction of corresponding U(IV) halides, [Cp′′ 2 UX2 ] (X = F, Cl, Br or I) with Na/Hg in toluene. The complexes were fully characterized analytically and single crystal analysis showed that Cl and Br form dimeric bridging compounds. In contrast, chlorine formed a trimeric bridge complex when it contained the ligand η5 -(CH3 )5 C5 (vide supra) [109]. This implies that (SiMe3 )2 C5 H3 is sterically more demanding than (CH3 )5 C5 . However, when the two structures [Cp′′ 2 U(µ-Cl)]2 and [Cp∗ 2 U(µ-Cl)]3 were compared, A it was found that the average U–C(Cp) distance is equivalent (2.78(2) ˚ A, respectively), whereas the U–Cl bond distance in the forand 2.77(1) ˚ A) was found to be significantly shorter than in the latter mer (2.810(4) ˚ ˚ (2.900(2) A). This may be due to weak U–Cl interaction in the latter, caused by the wider U–Cl–U angle of 154.9(1)◦ compared to that of the former 101.5(1)◦ . Investigating in this series, chlorine was found to form another bridging complex, [(Cp# )4 U2 (µ-Cl)2 ] (where Cp# = 1,3-(Me3 C)2 C5 H3 ) [120], which was produce by the reaction of K[1,3-(Me3 C)2 C5 H3 ] and UCl3 in THF (Eq. 27):

(27) The complex was also fully characterized by single crystal X-ray crystallography (Fig. 9) and found to have a similar structure to that of the [(Cp′′ )4 U2 (µ-Cl)2 ]. The average U–C distance of 2.79 ˚ A and U–Cp (centroid) distance of 2.51 ˚ A are not significantly different from those values in other trivalent uranium metallocenes. The reaction of (η5 -C5 H5 )3 U(THF) with dioxygen produces the bridged bimetallic complex [(η5 -C5 H5 )3 U]2 (µ-O) [121]. The analogous µ-sulfido complex was produced by the reaction of (η5 -C5 H5 )3 UCl with freshly prepared K2 S. A number of the dimeric complexes of the class {[η5 -1,3-R2 C5 H3 ]2 U (µ-X)}2 (R = Me3 Si or Me3 C) were structurally characterized [122] and found

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Fig. 9 The complex structure of [1,3-(Me3 C)2 C5 H3 ]4 U2 (µ-Cl)2 [120]. Reprinted with permission from IUCr Journals, http://journals.iucr.org

that they exist as dimers also in solution [123, 124]. These were found to react with Lewis bases to yield monomeric mono- or bis-ligand adducts [125–127]. In continuation to the exploration of bridging uranium complexes, Blake et al. reported a newer type of bimetallic bridging complex [128] of uranium with alkali metals of the type [UCp′′ 2 (µ-Cl)2 M(THF)2 ] (where M = Li(1), Na(2)), [UCp′′ 2 (µ-Cl)2 M(tmeda)] (where M = Li(3), Na(4)), [UCp′′ 2 (µ-Cl)2 Li(pmdeta)] (5), [UCp′′ 2 (µ-Cl)2 Li(THF)2 ] (6) and [PPh4 ] [UCp′′ ClX] (where X = Cl(7), Br(8); Cp′′ = η5 -(C5 H3 )–(SiMe3 )2 -1,3; THF =

Scheme 8 Synthetic routes for complexes 1–8 [128]

Metallocene Organoactinide Complexes

23

tetrahydrofuran; tmeda = (Me2 NCH2 )2 ; and pmdeta = (Me2 NCH2 CH2 )2 NMe). The complexes 1–5 were prepared by the reduction of [Cp′′ UCl2 ] with nbutyllithium or sodium amalgam in the presence of the appropriate neutral ligand, and 6–8 were prepared from [Cp′′ 2 U(µ-Cl)]2 [128] (Scheme 8). The single crystal X-ray structure of the complex [UCp′′ 2 (µ-Cl)2 Li(pmdeta)] was found to be very interesting and quite unique in nature because the Li center had a trigonal bipyramidal environment around it, with one Clligand axial and the other equatorial. The crystal structure of the complex [UCp′′ 2 (µ-Cl)2 Li(THF)2 ] has also been reported [129]. On investigating the reactivity of chalcogen donor ligands towards (Cp= )3 U(THF) (where Cp= = η5 -MeC5 H4 ), it was found that SCO, SPPh3 , SePPh3 , and TeP(n-Bu)3 form bridging complexes of the type [(Cp= )3 U]2 (µ-E) (where E = S, Se, Te) (Scheme 9) [130]. The complex [(Cp= )3 U]2 (µ-S) was characterized structurally (Fig. 10) and it was found that the two (Cp= )3 U units bridging by the S atom have a U–S–U bond angle of 164.9(4)◦ . The A supports the π-bonding explanation for average U–S distance of 2.60(1) ˚ the bridging-sulfido geometry [130]. The averaged U–C (ring) distance of A and the average ring centroid–U–ring centroid angle, 116(2)◦ 2.71±0.06 ˚

Scheme 9 Synthesis of chalcogen bridged uranium(IV) complexes [130]

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Fig. 10 Molecular structure of complexes [(MeC5 H4 )3 U]2 (µ-S) [130]. Reprinted with permission from [130];  (1986) American Chemical Society

is well within the range of complexes of the type Cp3 UIV X [111, 112, 114, 131, 132]. The linearity of the U–S–U bond angle was explained by assuming the bonding in this complex primarily as electrostatic, and the repulsive interaction between the (MeC5 H4 )3 U groups. An analogous bridging oxo complex was generated by the reaction of (Cpφ )3 U (where Cpφ = η5 -C5 H4 SiMe3 ) with CO2 or N2 O (Eq. 28) [133]: (28) The oxo bridged complexes {(Cpφ )3 U}2 (µ-O) and {(Cpφ )2 U(µ-O)}3 were also obtained simultaneously when [(Cpφ )3 U(OH)] was heated in toluene in the presence of an equivalent amount of [(Cpφ )3 UH]. Under the same reaction conditions, thermolysis of [(Cpφ )3 U(OH)] was found to afford only {(Cpφ )2 U(µ-O)}3 with the elimination of C5 H4 SiMe3 (trimethylsilylcyclopentadiene) [134, 135]. In a comparative reactivity study of Ce(III) and U(III) complexes with pyrazine, Ephritikhine and coworkers reported [136] that [Ce(C5 H4 R)3 ] forms a Lewis base type of adduct [Ce(C5 H4 R)3 (pyz)] while [U(C5 H4 R)3 ] oxidized to form dimeric complex [U(C5 H4 R)3 ]2 (µ-pyz), (where R = t-Bu, SiMe3 ; pyz = pyrazine) (Eq. 29):

(29)

Metallocene Organoactinide Complexes

25

2.5 Affinity Towards Lewis Bases As a part of the study on how steric and electronic factors influence the coordinative affinity of a given Lewis base towards trivalent uranium, a number of complexes were synthesized. The affinity of PMe3 was determined by synthesizing the complex [(C5 H4 Me)3 U(PMe3 )] according to the following reaction (Eq. 30):

(30) The complex [(Cp= )3 U(PMe3 )] is monomolecular in the crystalline state [114] and consists of an U atom coordinated to the three methylcyclopentadienyl groups in a pentahapto bonding mode and to the P atom of the trimethylphosphine molecule in a distorted tetrahedral arrangement. While comparing the structure of this complex with anaA log U(III) alkylphosphine structures, the U(III)–P distance of 2.972(6) ˚ A was found to be significantly shorter than the U–P distances of 3.211 ˚ A in {(CH3 C)5 }2 UH{(CH3 )2 PCH2 CH2 P(CH3 )2 } [137], 3.057 and and 3.092 ˚ 3.139 ˚ A in U(BH4 )3 {(CH3 )2 PCH2 CH2 P(CH3 )2 }2 [138], and 3.085 and 3.174 ˚ A in U(CH3 BH3 )3 {(CH3 )2 PCH2 CH2 P(CH3 )2 }2 [139]. On the other hand, the thiophene ligand SC4 H8 reacted with (Cp= )3 U(THF) in toluene to form a mononuclear complex (Cp= )3 U(SC4 H8 ) in solid state [112]. Like (Cp= )3 UPMe3 , here also the uranium atom is coordinated to Cp groups and to the sulfur atom of the tetrathiophene ligand in a distorted tetrahedral array. The structures of [(C5 H4 Me)3 U(SC4 H8 )] and (C5 H4 Me)3 U(THF) [111] are similar, but the U–S and U–O distances are A respectively. found to be 2.986 and 2.55 ˚ Another almost similar type of complex [(Cp= )3 U{4-(Me2 N)C5 H4 N}] was synthesized from the U(III)·THF adduct [113]. The complex was characterized structurally, and like other U(III) molecules it also has the tetrahedral arrangement around the uranium atom with three cyclopentadienyl centroids and a pyridine. The average Cp–U–Cp angle in [(Cp= )3 UL] (where L = 4-(Me2 N)C5 H4 N) is 117◦ which is almost the same as the values of 118◦ for L = SC4 H8 [112], 118◦ for OPPh3 [130], and 118◦ in CpU(OC4 H8 ) [111] but different form angles 106, 109 and 119◦ found in [(MeC5 H4 )3 UP(Me3 )], which can be explained with the help of the more crowding PMe3 molecule. Arliguie et al. [140] reported the synthesis of few uranium(III) thiolato complexes of the type Na[Cp∗ 2 U(SR)2 ] (where Cp∗ = η5 -C5 Me5 ; R = Ph, Me, i-Pr) by the reductive reaction of their corresponding uranium(IV) bis thiolate complexes with sodium amalgam. It is worth mentioning here that

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a similar type of reductive product could not be obtained for R = t-Bu, rather it gave U(IV) sulfide Na[Cp∗ 2 U(SBu-t)(S)]. Compound Na[Cp∗ 2 U(SPh)2 ] was thermodynamically stable but, in contrast, the complexes with R = Me and i-Pr were not so stable. In solution they slowly decompose to their analog sulfide derivatives Na[Cp∗ 2 U(SR)(S)] (R = Me and i-Pr). These facts suggested that compounds Na[Cp∗ 2 U(SR)2 ] (R = t-Bu, Me and i-Pr) undergo facile homolytic C–S bond cleavage of the SR ligand. From NMR studies, the C–S bond rupture was found to follow the order t-Bu > i-Pr > Me ≫ Ph. The complex Na[Cp∗ 2 U(SBu-t)(S)] was also alternatively synthesized by treating the U(III) chloride [Na(THF)1.5 ][Cp∗ 2 UCl2 ] [110] with NaSBu-t in THF. By using a bigger counter-cation like 18-crown-6-ether, the complex [Na(18-crown-6)][Cp∗ 2 U(SBu-t)S] [141] could have been recrystallized (Fig. 11). As usual, the uranium coordination geometry was pseudotetrahedral, which is typical of the Cp∗ 2 MX2 fragment. The U–S bond distances of 2.791(1) and 2.777(1) ˚ A are 0.1 ˚ A longer than in Cp∗ 2 U(SMe)2 , A, [142] in agreement with the difference of ionic radii between the 2.640(5) ˚ U(III) and U(IV) centers. In organometallic chemistry, the metal–ligand bond strength and the ligand displacement studies are fundamental problems. Brennan et al. [143] studied the relative affinity of Lewis bases towards (Me3 C5 H4 )3 U on the basis of the equilibrium constant and reported a ligand displacement series as PMe3 > P(OMe)3 > pyridine > tetrahydrothiophene ∼ tetrahydrofuran ∼ N(CH2 CH2 )3 CH > CO and towards (Me3 SiC5 H4 )3 U the series is EtNC > EtCN [144]. The observation that phosphite and isocyanide molecules,

Fig. 11 Crystal structure of [Na(18-crown-6)][Cp∗ 2 U(SBu-t)S]. Reproduced with permission from the Royal Society of Chemistry

Metallocene Organoactinide Complexes

27

which are generally classified as π-acceptors, are good ligands toward the trivalent uranium metallocenes suggests that the uranium center can act as a π-donor. Again for the phosphine and amines, the ligand displacement towards (RC5 H4 )3 U follows the order phosphine > amine [144]. To evaluate the U–P π-back bonding, complexes (MeC5 H4 )3 U[N(CH2 CH2 )3 CH] and (MeC5 H4 )3 U[P(OCH2 )3 CEt] were synthesized (Eq. 31) and characterized structurally. Table 3 shows a comparison of the M–L distance for the complexes of the type (RC5 H4 )3 U(L):

(31) Ephritikhine and coworkers investigated the reactivity of U(III) metallocene towards various pyridine-based azine molecules and reported the Lewis base adducts [(C5 H4 R)3 UL] (L = pyridine, 3-picoline, 3,5-lutidine, 3chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine). Except in the cases of L = 3-chloropyridine, pyridazine, pyrazine and s-triazine, the U(III) center was found to be oxidized (Scheme 10) [145].

Scheme 10 Reactivity of tris(cyclopentadienyl)U molecule towards various azines [144]

The complexes [(C5 H4 -t-Bu)3 UL] (L = pyridine, picoline) and [(C5 H4 SiMe3 )3 UL] (L = pyridine, lutidine, pyrimidine, and dimethylpyrazine) have been characterized by single crystal X-ray crystallography (Table 4). All the mononuclear complexes were found in the familiar pseudotetrahedral arrangement of the three η5 -cyclopentadienyl ligands and the coordinated Lewis base.

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Table 3 Some important bond length and angles of few of the cyclopentadienyl U(III) Lewis base complexes Complexes

Bond distance U–Cpav U–L (˚ A) (˚ A)

(MeC5 H4 )3 U[N(CH2 CH2 )3 CH] 2.82±0.03 (MeC5 H4 )3 U[P(OCH2 )3 CEt] 2.805 (MeC5 H4 )3 U(PMe3 ) 2.789 (MeC5 H4 )3 U(O=PPh3 ) 2.82±0.04

2.764(4) 2.521 2.972(6) 2.389(6)

Bond angle Refs. L–U–Cp1 L–U–Cp2 L–U–Cp3 (◦ ) (◦ ) (◦ ) 100.9 95.7 112.7 99.3

101.3 90.0 109.7 98.7

101.4 98.1 96.7 100.7

[144] [144] [114] [130]

2.6 Metal Ligand Back Donation In matrix isolation studies at cryogenic temperatures, it was observed that actinide carbonyl complexes, U(CO)6 , can exist below ca. 20 K. This led to studies on the synthesis of actinide carbonyl complexes, and in 1986 the first molecular actinide complex of carbon monoxide, (Me3 SiC5 H4 )3 UCO, was reported [146]. The coordination of CO to the metal center was found to be reversible. When a deep green solution of (Me3 SiC5 H4 )3 U in either pentane or hexane was exposed to CO at 1 atm and 20 ◦ C it turned to a burgundy colored complex (Me3 SiC5 H4 )3 UCO, which under vacuum or purging with argon gave back the original green colored complex. Examination of the IR spectrum of the burgundy solution using 12 CO showed νCO at 1976 cm–1 , with 13 CO νCO at 1935 cm–1 , and with 18 CO νCO at 1976 cm–1 [146]. The (Me3 SiC5 H4 )3 U also reversibly absorbs 12 CO in the solid state. Exposure of (Me3 SiC5 H4 )3 U in a KBr wafer to 12 CO at 1 atm results in the appearance of an absorption at 1969 cm–1 , which completely disappears when the sample is evacuated for 1.5 h. Using 13 CO (99%) causes the absorption to shift to 1922 cm–1 . To study the effect of substituents on the Cp ring, another few uranium carbonyl complexes with different substituted cyclopentadienyl ligands were synthesized [74, 147]. Both structural and spectroscopic studies indicate that in these complexes a strong degree of metal-to-ligand back donation occurs. The only complex which has been structurally characterized is (η5 -C5 Me4 H)3 U(CO) (Fig. 12), which shows evidence of metal-to-ligand back A. donation with the presence of short U–CCO bond distances of 2.383(6) ˚ Comparison of the υCO stretching frequencies for a series of compounds with different substituents on the ligand (Table 5) demonstrates that electrondonating groups on the ring increase the electron density at the metal center, increasing metal-to-ligand back donation. The νCO stretching lies in the order 1,3-(Me3 Si)2 C5 H3 > Me3 SiC5 H4 > Me3 CC5 H4 > C5 Me4 H, which indicates (C5 Me4 H)3 U is the best π-donor in this series of metallocenes.

∗

Centroid of the cyclopentadienyl ring

2.665(6) 2.683(8) 2.665(7) 2.646(4) 2.688(7) 2.656(6)

(C5 H4 –t-Bu)3 U(pyridine) (C5 H4 SiMe3 )3 U(pyridine) (C5 H4 –t-Bu)3 U(picoline) (C5 H4 SiMe3 )3 U(lutidine) (C5 H4 SiMe3 )3 U(pyrimidine) (C5 H4 SiMe3 )3 U(dimethylpyrazine)

2.84(7) 2.82(4) 2.83(7) 2.82(3) 2.81(3) 2.81(3)

Bond distance U–N U–C (˚ A) (˚ A)

Complexes

2.570(3) 2.55(1) 2.568(1) 2.55(2) 2.540(8) 2.54(2)

U–Cp∗ (˚ A)

Table 4 Comparative data for the trivalent uranium(III) complexes containing azine ligands

116.6–117.8 114.5–118.9 116.2–117.8 117.2–117.5 115.6–119.0 117.4–117.6

Bond angle Cp–U–Cp (◦ )

95.0–103.0 96.4–100.9 95.6–102.9 96.0–103.8 94.9–102.9 96.2–13.2

Cp–U–N (◦ ) [145] [145] [145] [145] [145] [145]

Refs.

Metallocene Organoactinide Complexes 29

30

M. Sharma · M.S. Eisen

Fig. 12 Molecular structure of (C5 Me4 H)3 U(CO) [147]. Reprinted with permission from [147];  (1995) American Chemical Society Table 5 υ(CO) frequencies of various U(III) complexes of the type [U(η5 -C5 H5–n Rn )3 CO] Complexes

State

νCO (cm–1 )

Refs.

(η5 -C5 Me4 H)3 U(CO) (η5 -Me3 CC5 H4 )3 U(CO) (η5 -Me3 SiC5 H4 )3 U(CO)

Nujol Hexane Hexane KBr Methylcyclohexane

1880 1960 1976 1969 1988

[74] [74] [74]

[η5 -(Me3 Si)2 C5 H3 ]3 U(CO)

[74]

For the heavier actinides the situation is little bit different as the energy of the 6d-orbitals drops across the series and hence the metal–ligand interactions become weaker. This is consistent with the report that plutonium forms less robust adducts compared to its lower actinide analogs. The com-

Metallocene Organoactinide Complexes

31

plex (η5 -C5 H5 )3 Pu(THF) could be isolated from solution, and the THF was removed by sublimation [53]; whereas in the analogous uranium compound, THF remains intact upon sublimation [55]. Different organic isocyanides adducts of uranium(III) metallocenes of the type [(R′ n C5 H5–n )3 U(CNR)] have been isolated [74], where R′ = H, Me, Me3 Si, Me3 C and R = Et; R′ = (1,3-(Me3 Si))2 and R = t-Bu; or R′ = Me4 and R = 4-(MeO)C6 H4 , CH3 , i-Pr, t-Bu and 2,6-Me2 C6 H3 . All of the isocyanide complexes were made by the addition of an excess of CNR to the [(R′ n C5 H5–n )3 U] complex in hexane, toluene or diethyl ether (Eq. 32):

(32) All the isocyanides have 1 : 1 stoichiometry with the exception of MeC5 H4 and R = 2,6-Me2 C6 H3 for which both the 1 : 1 and 1 : 2 adducts were obtained. Table 6 lists the isocyanide complexes and the νCNR frequencies in the infrared spectrum. The IR spectra showed that νCN increased slightly for the alkyl isocyanide complexes and decreased slightly for the aryl isocyanide complexes relative to νCN for the free ligands. The substituents on the cycloTable 6 IR data of different cyclopentadienyl U(III) isocyanide complexes [74] Complex

νCN a

(C5 H5 )3 U(CNEt) (MeC5 H4 )3 U(CNEt) (MeC5 H4 )3 U(CNXyl) (MeC5 H4 )3 U(CNXyl)2 (Me3 SiC5 H4 )3 U(CNEt) (Me3 CC5 H4 )3 U(CNEt) {1,3-(Me3 Si)2 C5 H3 }3 U(CNBu-t) (Me4 C5 H)3 U(CNMe) (Me4 C5 H)3 U(CNPr-i) (Me4 C5 H)3 U(CNBu-t) (Me4 C5 H)3 U(CNC6 H4 –p–OMe) (Me4 C5 H)3 U(CNXyl)

2170 2155 2060 2095 2178 2180 2140 2165 2143 2127 2072 2052

a

In nujol mull, cm–1

32

M. Sharma · M.S. Eisen

pentadienyl ligand also affect νCN , and for a given isocyanide the νCN values follow the order Me3 C ≈ Me3 Si ≈ (Me3 Si)2 > H > Me > Me4 . To compare the νCN stretching frequencies between the 4f and 5f series it is worth mentioning here that for a given adduct, νCN for the uranium complex is always less than that of its cerium analog. This comparison clearly shows that uranium in the trivalent complexes is a better π-donor than its cerium analog. 2.7 Beyond the Tris-Cp Complexes There exist relatively few examples of trivalent actinide complexes with two cyclopentadienyl rings. Compounds of the parent cyclopentadienyl ion are somewhat rare. Examples include the reported compounds (η5 -C5 H5 )2 ThCl [52] and (η5 -C5 H5 )2 BkCl [59] that exist as dimers. The compounds (η5 -C5 H4 Me)2 NpI(THF)3 and (η5 -C5 H4 Me)NpI2 (THF)3 were prepared by reactions of NpI3 (THF)4 with Tl(C5 H4 Me) in tetrahydrofuran [148]. A cationic bis(pentamethylcyclopentadienyl)uranium(III) complex has been reported by Ephritikhine and coworkers [149]. The complex [(η5 -C5 Me5 )2 U(THF)2 ][BPh4 ] is generated by the protonation of the complex (η5 -C5 Me5 )2 U[N(SiMe3 )2 ] with [NH4 ][BPh4 ]. Cendrowski-Guillaume et al. reported the synthesis of a mixed cyclopentadienyl/cyclooctatetraenyl complex [U(COT)(Cp∗ )(HMPA)] (where COT = η-C8 H8 , HMPA = OP(NMe2 )3 ) [150] by the treatment of [U(COT)(HMPA)3 ] [BPh4 ] with KCp∗ . The complex [U(COT)(Cp∗ )(HMPA)] was characterized crystallographically and was found to adopt trigonal configuration as shown

Fig. 13 Molecular structure of [U(η-C8 H8 )(Cp∗ ){OP(NMe2 )3 }] [150]. Reproduced with permission

Metallocene Organoactinide Complexes

33

in Fig. 13. The U–O, U–COT(centroid) and U–Cp∗ (centroid) bond distances A, respectively. were found to be 2.461(8), 2.01(1), and 2.50(1) ˚ Recently some mono- and bis(cyclopentadienyl) compounds [(C5 H4 –t-Bu)UI2 ] and [(C5 H4 –t-Bu)UI] were synthesized by comproportionation reactions of [U(C5 H4 –t-Bu)3 ] and [UI3 (L)4 ] (where L = THF or py) in the molar ratio of 1 : 2 and 2 : 1, respectively. The treatment of [UI3 (py)4 ] with one or two molar equivalents of LiC5 H4 –t-Bu in THF afforded the [(C5 H4 –t-Bu)UI2 ] and [(C5 H4 –t-Bu)2 UI2 ] compounds, respectively (Scheme 11) [151].

Scheme 11 Synthesis of various mono- and di-cyclopentadienyl complexes by comproportionation reaction [150]

The complex [U(C5 H4 –t-Bu)I2 (py)3 ] was characterized by X-ray crystallography and the average U–C, U–I and U–N bond distances were found to be 2.80(5), 3.17(2), and 2.66(5) ˚ A, respectively. On investigating the affinity of cyclopentadienyl ligand, (C5 H4 -t-Bu), towards Ln(III) (Ln = La, Ce, Nd) and U(III), it was reported that the U(III) center is relatively more prone to coordinate the cyclopentadienyl ligand [151]. Investigating the mix ligand complexes, Summerscales et al. discovered that the U(III) COTR /Cp mixed sandwich complex [U(η-C8 H6 {Si-i-Pr3 -1,4}2 ) (Cp∗ )(THF)] induces efficient cyclotrimerization of CO to give [U(η-C8 H6 {Si-i-Pr3 -1,4}2 )(Cp∗ )]2 (µ-η1 :η2 -C3 O3 ) [152]. Roger et al. reported [153] the synthesis of mixed cyclopentadienyl/dithiolene complexes and compared the structural parameters with the analog lanthanide complexes. The treatment of [(Cp∗ )2 UCl2 ] with Na2 dddt in THF afforded the complex [(Cp∗ )2 UCl(dddt)Na(THF)2 ], which upon treatment in toluene afforded the salt-free compound [(Cp∗ )2 U(dddt)] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). Reduction of [(Cp∗ )2 U(dddt)] with Na/Hg or addition of Na2 dddt to [(Cp∗ )2 UCl2 Na(THF)x ] in the presence of 18-crown-6 gave [Na(18-crown-6)(THF)2 ][(Cp∗ )2 U(dddt)]. The crystal structures of [(Cp∗ )2 U(dddt)], [Na(18-crown-6)(THF)2 ][(Cp∗ )2 U(dddt)] ·THF were determined by X-ray diffraction analysis and a few selected parameters around the U center are given in Table 7. To carry out a comparative study of structural features, magnetic properties, and reactivities of the lanthanides(III) and actinides(III), Mehdoui et al. [154] reported the synthesis of complexes of the type [(Cp∗ )2 MI(bipy)]

34

M. Sharma · M.S. Eisen

Table 7 Selected crystal data, bond lengths (˚ A) and bond angles (◦ ) in [(Cp∗ )2 U(dddt)] and [Na(18-crown-6)(THF)2 ][(Cp∗ )2 U(dddt)]·THF [153]

Crystal system Space group U–S(1) U–S(2) U–Cp∗ (av) Cp∗ –U–Cp∗a S(1)–U–S(2) a

[(Cp∗ )2 U(dddt)]

[Na(18-crown-6)(THF)2 ][(Cp∗ )2 U(dddt)]·THF

Orthorhombic Pnma 2.629(3) 2.650(3) 2.73(2) 133.1 78.93(12)

Triclinic PI 2.7807(16) 2.7661(17) 2.79(2) 135.7 76.24(5)

Centroid

(where M = Ce, U; bipy = 2, 2′ -bipyridine) by the treatment of [(Cp∗ )2 CeI] or [(Cp∗ )2 UI(py)] with one equivalent of bipy in THF. These complexes were found to be further transformed into [(Cp∗ )2 M(bipy)] (where M = Ce, U) by Na(Hg) reduction. On the other hand, the reaction of [(Cp∗ )2 CeI] or [(Cp∗ )2 UI(py)] with one equivalent of terpy (terpy = 2, 2′ :6′ , 2′′ -terpyridine) in THF afforded the ionic complex [(Cp∗ )2 M(terpy)]I (where M = Ce, U), which on reduction by Na(Hg) afforded the neutral complexes [(Cp∗ )2 M(terpy)] (where M = Ce, U) [154]. The uranium(III) complexes [η5 -1,3-R2 C5 H3 ]3 U react stoichiometrically with one equivalent of water to produce complexes of the type {[η5 -1,3-R2 C5 H3 ]2 U(µ-OH)}2 (where R = Me3 Si or Me3 C) [155], which upon heating undergo an unusual “oxidative elimination” of hydrogen to yield the corresponding µ-oxo complexes. The kinetics of this process has been examined, and the reaction was found to be intramolecular, probably involving a stepwise α-elimination of hydrogen. The triiodide complex UI3 (THF)4 was found to be a valuable starting reagent in generating mono(cyclopentadienyl) uranium(III) complexes [156]. Reaction of one equivalent of UI3 (THF)4 with K(C5 Me5 ) results in the formation of the complex (η5 -C5 Me5 )UI2 (THF)3 (9). In the solid state this complex (9) exhibits a pseudo-octahedral mer, trans-geometry, with the cyclopentadienyl group occupying the axial position:

Metallocene Organoactinide Complexes

35

In the presence of excess pyridine, this complex can be converted to the analogous pyridine adduct, (η5 -C5 Me5 )UI2 (py)3 . The complex (η5 -C5 Me5 ) UI2 (THF)3 was found to be very reactive and generated the bis(ring) product (η5 -C5 Me5 )2 UI(THF) by reaction with K(C5 Me5 ), or with two equivalents of K[N(SiMe3 )2 ] produced (η5 -C5 Me5 )U[N(SiMe3 )2 ]2 . The solid state structure of the bis(trimethylsilyl) amide derivative reveals close contacts between the A). uranium center and two of the methyl carbons (2.80(2), 2.86(2) ˚ Oxidation of (η5 -C5 Me5 )UI2 (THF)3 with CS2 or ethylene sulfide produces a complex of the formula [(η5 -C5 Me5 )UI2 (THF)3 ]2 (S). This species undergoes slow decomposition in solution to yield a polynuclear complex (10) [157]:

3 Tetravalent Chemistry It is not surprising that the cyclopentadienyl ligands also dominate the tetravalent chemistry of the early actinide elements. Complexes of the type (η5 -C5 H5 )4 An (where An = Th [158], Pa [159], U [42], and Np [160]) were the earliest actinide complexes with the tetrakis(cyclopentadienyl) ligand. Among these, the uranium and thorium compounds have been structurally characterized [161, 162]. Moreover, IR spectral and X-ray powder data confirm that all four complexes are isostructural. (η5 -C5 H5 )4 U is found to be A. This psuedotetrahedral, with a mean U–Cring bond distance of 2.81(2) ˚ is somewhat longer than average U–Cring distances for other U(IV) cyclopentadienyl complexes and reflects a degree of steric crowding. In 1986, Rebizant and coworker reported a related thorium complex containing the tetrakis(indenyl) ligand [163], in which the thorium atom is not bonded in η5 fashion to the carbons of the five-membered ring portion of the indenyl ligand. Once a complex has been synthesized, the next major challenge for the researcher is to find out the nature of bonding. Working on this, Burns et al. reported that in comparison to the lanthanides these complexes ex-

36

M. Sharma · M.S. Eisen

hibits more covalency in chemical bonding; unlike lanthanides they do not react with FeCl2 to form ferrocene, but are still believed to be more ionic than the majority of d-transition metal cyclopentadienyl complexes [78]. The isolation of these tetrakis(cyclopentadienyl) complexes carved the path for the researchers to peep inside the chemistry of the actinide(IV) complexes. Reynolds and Wilkinson [2] were the pioneers reporting the preparation of (η5 -C5 H5 )3 UCl, the first complex of the type Cp3 AnX, by the reaction of uranium tetrachloride with sodium cyclopentadienide in tetrahydrofuran. Following this, the chemistry was extended to prepare other actinide complexes of this class [164, 165]. Later, alternative routes were also reported for the synthesis of the complex (η5 -C5 H5 )3 UCl by the reaction of actinide halides with cyclopentadienyl thallium in DME (Eq. 33) [166]: (33) Following this, to study the bonding and geometry of these kind of organoactinide complexes, a plethora of complexes of the type [(Rn C5 H5–n )3 AnX] were synthesized and characterized structurally. Most of the tris(cyclopentadienyl)uranium halides were prepared by the reaction of uranium tetrachloride and a stoichiometric amount of either sodium or potassium salt of an appropriately substituted cyclopentadiene, generated in situ [2, 167–169]. The drawbacks of this procedure were the lack of careful control of stoichiometry and polymer formation resulting from the use of excess cyclopentadiene in the preparation of the alkali metal salt. To overcome these drawbacks, Anderson et al. [170] reported the synthesis of (C5 H5 )3 UCl by the reaction of uranium tetrachloride and a stoichiometric amount of thallous cyclopentadienide in a suitable solvent. Later, a similar method was used to prepare (C5 H4 CH2 C6 H5 )3 UCl [171]. In an attempt to obtain hydrocarbon-soluble complexes of thorium and uranium, bulky substituted tris(cyclopentadienyl)thorium(IV) or uranium(IV) halides were prepared by the ready transmetallation between MCl4 (M = Th or U) and the appropriate lithium cyclopentadienyl [172]. On investigating the limiting factors of steric hindrance around the actinide metal center and sterically induced reduction chemistry, Evans et al. [173] could have synthesized Cp∗ 3 UCl, by the controlled reaction of Cp∗ 3 U with one equivalent of PhCl (Eq. 34):

(34) The molecule Cp∗ 3 UCl is considered to be a sterically highly crowded complex since the U4+ center is relatively small and is bonded to four ligands. Upon addition of another equivalent of PhCl, the complex Cp∗ 3 UCl subse-

Metallocene Organoactinide Complexes

37

quently forms Cp∗ 2 UCl2 over several days (Eq. 34). Once the existence of the Cp∗ 3 UCl was established, several synthetic routes were developed [173, 174], as shown in Scheme 12. The fluorine analog of this complex was also isolated and readily synthesized by the reaction of Cp∗ 3 U with HgF2 . Although the bromide and iodide analog of this complex have been synthesized by the reaction of Cp∗ 3 U with PhX (X = Br, I) and analyzed spectroscopically, they have not yet been characterized crystallographically [173].

Scheme 12 Various synthetic routes towards Cp∗ 3 UCl [173, 174]

The existence and isolation of (C5 Me5 )3 UCl (Fig. 14) clearly indicates that this class of complexes has not yet reached the limits of steric crowding. The structure of (C5 Me5 )3 UCl was found to be rather similar to that of (C5 Me5 )3 U, as shown by the overlay of the two structures in Fig. 15 [94]. From Fig. 15 it is evident that in (C5 Me5 )3 U [85] and (C5 Me5 )3 UCl the ring centroids are coplanar, and that they also crystallize in the same space group P63 /m. A molecular mirror plane bisects the three symmetry-equivalent C5 Me5 rings, and the chloride ligand is disordered on either side. The A and U–C(C5 Me5 ) bond distances fall within the range 2.780(6)–2.899(9) ˚ average 2.833(9) ˚ A. They are equivalent within experimental error to those A) and (C5 Me4 H)3 UCl (2.791(12) ˚ A) [175]. Thus, of (C5 Me5 )3 U (2.857(4) ˚ the chloride ligand does not appear to perturb the U–C(C5 Me5 ) parameA is exceptionally longer than those in ters, but the U–Cl bond of 2.90(1) ˚ A) and in (C4 Me4 P)3 UCl (2.67(1) ˚ A) [85, 176] (C5 Me4 H)3 UCl [175] (2.637 ˚ (Table 8). Apart from the cyclopentadienyl ligand, complexes with other ligands have also been reported. The tris(indenyl)uranium and tris(indenyl)thorium

38

M. Sharma · M.S. Eisen

Fig. 14 Crystal structure of (η5 -C5 Me5 )3 UCl [173]. Reprinted with permission from [173];  (2000) American Chemical Society

complexes have been prepared by the metathesis reactions with K(C9 H7 ) in THF [167, 177–179], but in this review we have decided to restrict our discussion only to the cyclopentadienyl complexes. The molecular structure of several tri(cyclopentadienyl)AnX complexes have been structurally determined. The structure of these complexes was found to possess pseudotetrahedral geometry, with the halide ligand on an approximate threefold axis of symmetry. To study the structural correlativity, the average M–C and M–X bond distances are tabulated (Table 8) for most of the common complexes of this class. The An–C and An–X bond lengths are consistent in most of the complexes for a particular metal center and in com-

Fig. 15 Overlay drawing of (C5 Me5 )3 U (- -) and (C5 Me5 )3 UCl(-) [94]. Reprinted with permission from [94];  (2002) American Chemical Society

2.74 2.72(1) 2.73(3) 2.733(1) 2.77(1) 2.79(1) 2.829(6) 2.833(9) 2.84(1) 2.84(2) 2.85(1) 2.83(1)

(η5 -C5 H5 )3 UCl (η5 -C5 H5 )3 UBr (η5 -C5 H5 )3 UI (η5 -C5 H4 CH2 Ph)3 UCl [η5 -(Me3 Si)2 C5 H3 ]3 UCl (η5 -C5 Me4 H)3 UCl (η5 -C5 Me5 )3 UF (η5 -C5 Me5 )3 UCl [η5 -(Me3 Si)2 C5 H3 ]3 ThCl [η5 -(Me3 Si)2 C5 H3 ]2 (C5 Me5 )ThCl [η5 -(SiMe2 –t-Bu)2 C5 H3 ]3 ThCl {η5 -[(Me3 Si)2 CH]C5 H4 }3 ThCl 2.559(16) 2.820(2) 3.059(2) 2.627(2) 2.614(2) 2.637 2.43(2) 2.90(1) 2.651(2) 2.657(5) 2.648(2) 2.664(2)

Bond length M–CCp (average) M–X (˚ A) (˚ A)

Complexes

Table 8 Structural information for Cp3 AnX complexes

101.28 101(1) 101.3(6) 101.2 116.6 98.45 90.00 90.00 116.7 97.8 99.4 100.7

Bond angle Cp(2)–U–L (◦ ) 101.42 99.7(4) 100.9(6) 99.9 118.0 98.45 90.00 90.00 117.6 97.3 99.7 101.6

Cp(1)–U–L (◦ )

99.30 101.0(8) 100.0(6) 98.8 116.4 98.45 90.00 90.00 116.6 99.8 100.9 97.0

Cp(3)–U–L (◦ ) [67] [168] [169] [171] [128] [175] [173] [173] [172] [172] [172] [172]

Refs.

Metallocene Organoactinide Complexes 39

40

M. Sharma · M.S. Eisen

parison to U, Th–C and Th–X bond lengths are slightly longer, as expected from their large ionic radii. Among these, the smallest M–X bond length was found for the complex Cp∗ 3 UF, although it seems to be a sterically very crowded molecule. This difference may be due to the smaller size and higher electron affinity of the F atom. Discovery of this interesting branch of uranium chemistry has given impetus to develop a similar type of chemistry for the Th(IV) system, as it may be also very interesting to the SIR (sterically induced reduction) processes. However, the synthesis of Cp∗ 3 ThX-type complexes was not so easy as the precursor Th(III) is much less accessible. Ultimately, the complex Cp∗ 3 ThH could have been synthesized (Eq. 35) and characterized crystallographically. The crystal structure reveals that it crystallized in the same space group (P63 /m) as that of the complexes Cp∗ 3 UX (X = Cl, F). The Th–ring centoids distance lies almost in the same range as that of Cp∗ 3 UX and Cp∗ 3 U. The Th–H bond distance was found to be 2.00(13) ˚ A, which is the shortest among all the other M–X (where M = U, Th; X = halides) bonds of the similar types of tris-cyclopentadienyl uranium or thorium halide complexes. Later, Berthet et al. reported the synthesis of thermally stable U(IV) hydride complexes, [{η5 -(Me3 Si)C5 H4 }3 UH] and [{η5 -(Me3 C)C5 H4 }3 UH] [180]:

(35) The existence of the crowded molecule Cp∗ 3 UX, catalyzed the researchers to find more facts of such type of complexes. A number of derivatives of the class Cp3 AnY (where An = U, Np, Pu; Y = CN, CNBH3 , NCS) have been generated either by prototype reactions, include protonation of (η5 -C5 H5 )4 U or by metathesis reaction of (η5 -C5 H5 )3 AnCl (Eqs. 36–37) [121, 181–187]:

(36)

(37)

Metallocene Organoactinide Complexes

41

In the case of the cyanide complexes, the metal–ligand bond is found to be very stable as the reaction of (η5 -C5 H5 )3 UCl with KCN may be carried out in water [186]. The ionization of (η5 -C5 H5 )3 UCl in water yielded the five-coordinate adduct [(η5 -C5 H5 )3 U(H2 O)2 ]+ [188]. The isolation of this complex opened the door for the researchers to investigate more about fivecoordinated species like [(η5 -C5 H5 )3 An(XY)]– . Bagnall et al. reported the successful synthesis of [(η5 -C5 H5 )3 An(NCS)2 ]– (where An = U, Np, Pu) with a sufficiently large cation like [K(Crypt)] (Crypt = cryptofix-222), NMe3 , or AsPh4 [186]. On the basis of spectrophotometric and other evidences a trigonal-bipyramidal coordination for the metal atoms has been proposed. This proposed structure was further supported by structural characterization of neutral base adducts such as (η5 -C5 H5 )3 U(NCS)(NCMe) [189, 190] or (η5 -C5 H5 )3 U(NCBH3 )(NCMe) [191]. The geometry of these complexes found to be trigonal-bipyramidal, in which the smaller ligands adopt the axial positions. While investigating this series of complexes, the cationic species [(η5 -C5 H5 )3 U(NCR)2 ]+ (where R = CH3 , C2 H5 , n-C3 H7 or Ph) were able to isolate as a [BPh4 ]– salt by the reaction of (η5 -C5 H5 )3 UCl with Na[BPh4 ] in water/NCR mixtures [190]. Applying the protonolysis reaction to tris(cyclopentadienyl)diethylaminouranium, [(Cp)3 U(NEt2 )], with [NHEt3 ][BPh4 ] in THF yielded the cationic complex [(Cp)3 U(THF)]BPh4 (Scheme 13) [192], which was easily transformed to its chloride derivative [(Cp)3 UCl] by the addition of NBu4 Cl. Apart from these, a number of mono- and di-cationic species containing mono-, bis-cyclopentadienyl as well as mixed cyclopentadienyl and cyclotetraene complexes have been synthesized.

Scheme 13 Synthesis of various cationic uranium complexes [192]

The reaction of tris(cyclopendienyl)uranium chloride with potassium thiocyanate and potassium phenoxide in THF produced [(η5 -C5 H5 )3 U(NCS)] [193] and [(η5 -C5 H5 )3 U(C6 H5 O)] [194], respectively (Scheme 14). The complexes were characterized crystallographically and the bond distances between U–N and U–O were found to be 2.34(2)

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M. Sharma · M.S. Eisen

A, respectively. Although the complex [(η5 -C5 H5 )3 U(C6 H5 O)] and 2.119(7) ˚ seems to be sterically more hindered, still the U–O bond distance is shorter. The placement of the phenyl ring in it divides the Cp rings into two classes. Two Cp rings are non-planar with the phenyl moiety, while the third one is almost planar (Fig. 16). Complexes [(η5 -C5 H5 )3 UNCBH3 ] and [(η5 -C5 H5 )3 UNCB(C6 H5 )3 ] were synthesized [184] by the reaction of [(η5 -C5 H5 )3 UCl] with the corresponding anionic borates (Scheme 14). The complexes were characterized spectroscopically and found that they bind to the metal center through the N-donor site.

Scheme 14 Various derivatives of Cp3 UCl [184, 193, 194]

By using pyridinium triflate (pyHOTf) the protonolysis to U–C and U–N bonds in [Cp3 UR] (where R = NEt2 , n-Bu) afforded triflate complexes [(Cp)3 U(OTf)] and [(Cp)2 U(OTf)2 (py)]. Even with the precursor [(Cp∗ )2 UMe2 ], [(Cp∗ )2 U(OTf)2 ] (OTf = O3 SCF3 ) was yielded [195–197]. The complex [(Cp∗ )2 U(OTf)2 ] crystallized from THF-pentane solvent system as [(Cp∗ )2 U(OTf)2 (OH2 )] and was found to have the usual bent-sandwich configuration with an unsymmetrical arrangement of OTf and H2 O ligands in the equatorial position. In the presence of an excess of t-BuCN, the complex

Metallocene Organoactinide Complexes

43

Fig. 16 Crystal structure of [(η5 -C5 H5 )3 U(C6 H5 O)] [194]. Reprinted with permission from IUCr Journals, http://journals.iucr.org

[(Cp)3 U(OTf)] was transformed into [Cp3 U(OTf)(CNBu-t)]. Interestingly, the triflate group was not displaced by the isocyanide molecule (Fig. 17). It has been found that Cp3 AnX undergo metathesis and protonation reaction with various ligands like alkoxide (OR), amide (NR2 ), phosphide (PR2 ), and thiolate (SR) groups to generate the complexes of the type Cp3 An(L) [142, 164, 198–201]. The alkyl thiolate complex [(Cp∗ )3 Th(SCH2 CH2 CH3 )2 ] was synthesized by the reaction of Cp∗ 2 ThMe2 with HS(n-Pr) in toluene. It crystallizes in the monoclinic space group C2/c with Th–C bond disA [202]. The complexes of the type Cp3 U(IV) (SR) were pretance 2.718(3) ˚ pared by two principal methods namely: (1) substitution of the chloride

Fig. 17 Crystal structure of [Cp3 U(O3 SCF3 )(CNBu-t)] [196]. Reproduced with permission from the Royal Society of Chemistry

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M. Sharma · M.S. Eisen

group of [(Cp)3 UCl] by SR– , and (2) oxidation of the trivalent precursors [(Cp)3 U(THF)]. The method (1) was unsuccessful for the complexes containing the substituted cyclopentadienyl ligands (C5 H4 SiMe3 ) and (C5 H4 Bu-t), hence they were treated with the disulfide RSSR (where R = Me, Et, iPr, t-Bu or Ph). Similarly, [(Cp)3 U(SeMe)] and [(C5 H4 SiMe3 )3 U(SeMe)] were afforded by the treatment of MeSeSeMe with the corresponding U(III) species [142]. Like most of the tris(cyclopentadienyl) complexes of the type Cp3 UX, the crystal structure of [(Cp)3 U(SMe)] (Table 9) adopted pseudoteA) and the U–S–C(1) trahedral geometry. The U–S bond length (2.696(4) ˚ angle, (107.2(5)◦ ) were found to be almost similar to those in other uranium thiolate complexes [203–210]. While discussing the reactivity of [(Cp)3 U(SPr-i)], it was found that the U–S bond was readily cleaved by various ligands, as shown in Scheme 15.

Scheme 15 Reactivity of [(Cp)3 U(SPr-i)] [142]

Anderson et al. reported [73] the synthesis of the similar type of complexes [(MeC5 H4 )3 U(SPr-i)] and [(Me3 CC5 H4 )3 U(SPh)] by the reaction of (MeC5 H4 )3 U(THF) with isopropyl thiol and (Me3 CC5 H4 )3 U with HSPh, respectively. By applying the Mössbauer spectroscopy technique the bonding nature of the Np(IV)–ligand bond was determined for a number of complexes of the type Cp3 NpOR, Cp3 NpR (R = alkyl), and Cp3 NpAr (Ar = aryl) [164]. Strong σ character for the Np–(n-Bu) and Np–(C6 H4 C2 H5 ) bonds were found, while in Np–OR it was less pronounced. An interesting class of complexes, (η5 -C5 H5 )3 AnR (An = Th, U, Np), where R is an alkyl or aryl group, were synthesized by the ligand substitution reaction of (η5 -C5 H5 )3 AnX (X = halide) with Grignard or alkyllithium reagents. An extensive study was carried out by various research groups on the alkyl complexes [174, 211–218]. Many of the complexes have been characterized crystallographically and found to have pseudotetrahedral geometry (Table 10). The coordination environment of these

Metallocene Organoactinide Complexes

45

Table 9 Important bond distances in actinide complexes containing O- or N- or S-donor ligands Complex

Bond distance (˚ A) U–Cp(I) U–Cp(II) U–Cp(III) U–heteroatom centroid centroid centroid

Refs.

[(Cp)3 U(NCS)] [(Cp)3 U(C6 H5 O)] [(Cp)3 U(O3 SCF3 )(CNBu-t)]

2.45(4) 2.45(2) 2.479

2.51(4) 2.47(2) 2.484

2.47(4) 2.48(2) 2.482

[193] [194] [196]

[(Cp)2 U(O3 SCF3 )2 (OH2 )]

2.439

2.469

–

[(η5 -C5 H5 )3 U(SMe)] [(Cp)2 U(O3 SCF3 )2 (py)2 ]

2.468 2.477 2.454(5) 2.466(5)

2.477 –

U–N 2.34(2) U–O 2.119(7) U–O 2.36(1) U–C 2.59(2) U–O 2.36(1) U–O 2.40(1) H–OH 2 O 2.57(2) U–S 2.696 U–O 2.395(4) U–O 2.385(4) U–N 2.685(5) U–N 2.614(5)

[196]

[142] [197]

Table 10 Some bond lengths of uranium(IV) complexes containing U–C σ-bond Complex

Bond distance (˚ A) U–Cp U–Y average

Refs.

[(Cp)2 U{µ-CHP(Ph)2 (CH2 )}]2

2.53 2.46 2.51 2.52

[236]

[(Cp)3 UCHP(CH3 )2 (Ph)] [(Cp)3 U{(CH3 )C(CH2 )2 }] (Cp)3 U(n-C4 H9 )

2.79(3) 2.74(1) 2.728(12) 2.738(15) 2.747(14) 2.705(7) 2.742(5) 2.73(5) 2.418 ∗ 2.81

U1 –C2 2.67(4) U1 –C3 2.44(4) U1 –C4 2.55(3) U2 –C1 2.66(4) U2 –C3 2.48(4) U2 –C4 2.41(4) U–C 2.29(3) U–C 2.48(3) U–C 2.426(23)

U–C 2.541(15)

[117]

2.339 2.66(2)

[217] [174] [218]

(Cp)3 U[CH2 (p-CH3 C6 H4 )] (Cp)3 U(CCH) (η5 -C5 Me5 )3 UMe (Cp)4 U ∗

centroid

[238] [219] [117]

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Fig. 18 Crystal structure of (η5 -C5 H5 )3 U[CH2 C(CH3 )2 ] [219]. Reprinted with permission from [219];  (1975) American Chemical Society

complexes was found to be almost saturated as the allyl ligands can only be accommodated in a simple σ-bonded fashion [219], as shown in Fig. 18. In this class of compounds, particularly with uranium, the metal–carbon bond possesses considerable ionic character. This is evident from the formation of Cp3 UOCH3 by the reaction of methanol with Cp3 UR, which further confirms the existence of a metal–carbon σ-bond. Marks et al. [214] used NMR spectroscopy to study the nature of bonding in the class of complexes Cp3 UR (where R = CH3 , allyl, neopentyl, C6 F5 , i-C3 H7 , n-C4 H9 , t-C4 H9 , cis2-butenyl, trans-2-butenyl, C6 H5 , vinyl). The allyl compound Cp3 U(allyl) was found to adopt the monohapto geometry in ground state at low temperature, but at room temperature it shows fluxional behaviors, presumably interconverting sites by means of π-bonding intermediates. Most of these complexes Cp3 UR (where R = CH3 , allyl, neopentyl, C6 F5 , i-C3 H7 , n-C4 H9 , t-C4 H9 , cis2-butenyl, trans-2-butenyl, C6 H5 , vinyl) were found to have high thermal stability, which in turn depends on the nature of the R group. Based on kinetic studies in toluene solution, a general stability order was proposed as: primary > secondary > tertiary [214]. Another interesting σ-bonded organouranium complex containing acetylene or another organometallic moiety has been reported by Tsutsui et al. [215]. Complexes (Cp)3 U(C ≡ CH), (Cp)3 U(C5 H4 )Fe(C5 H5 ), (Cp)3 U(C5 H4 )Fe(C5 H4 )U(Cp)3 and (Cp)3 U(p – C6 H4 )U(Cp)3 were synthesized by following the simple metathesis reactions shown in Eqs. 38–41:

(38)

Metallocene Organoactinide Complexes

47

(39)

(40)

(41) The complexes with ferrocene were found to be thermally stable in vacuo to at least 180 ◦ C. The decomposed products were only ferrocene, cyclopentadiene, and uranium. This results further supports the mode of decomposition by the proton abstraction by R from a Cp group as suggested by Marks [214]. Complexes of the type (η5 -C5 H5 )3 U(EPh3 ) (where E = Si [220, 221], Ge [222]) were prepared by the reaction of (η5 -C5 H5 )3 UCl with Li(EPh3 ). The stannyl analog (η5 -C5 H5 )3 U(SnPh3 ) was made from the protonolysis of (η5 -C5 H5 )3 U(NEt2 ) with HSnPh3 or by the transmetallation reaction of HSnPh3 with (η5 -C5 H5 )3 U(EPh3 ) (where E = Si, Ge) [221]. The silyl compound was found to be very reactive and can easily be converted into (η5 -C5 H5 )3 U(OSiPh3 ). The η2 -iminoacyl complexes [(η5 -C5 H5 )3 U {C(EPh3 ) = N(xylyl)}] (E = Si, Ge) were generated by the insertion of xylylisocyanide into the U–E bonds. The study for the migratory insertion reaction of CO to a series of thorium hydrocarbyls complexes [(η5 -C5 H5 )3 ThR] (where R = i-C3 H7 , sec-C4 H9 , neo-C5 H11 , n-C4 H9 , CH2 Si(CH3 )3 , CH3 , and CH2 C6 H5 ) shows that η2 -acyl insertion products, (C5 H5 )3 Th(η2 -COR), [223] can be obtained when R = i-C3 H7 , CH3 , n-C4 H9 , sec-C4 H9 and neo-C5 H11 (Scheme 16). The structure of these complexes can best be described with the help of a “carbene like” resonance form A and B (Scheme 16). But when R = i-C3 H7 or CH2 Si(CH3 )3 enolate rearrangement products were isolated (Scheme 16). The relative rates of insertion for the ligands were found to follow the order i-Pr > s-Bu > neo-C5 H11 > n-Bu > CH2 Si(CH3 )3 > Me > CH2 C6 H5 . The relative rates of CO insertion reflect both steric and electronic effects with significant correlation to the

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Th-R bond disruption enthalpies. When this correlation was compared with the CO2 insertion, to generate carbonate complexes, it showed that carboxylation is significantly slower than carbonylation, and exhibits different trends on the dependence of rate on the alkyl ligand [223].

Scheme 16 CO insertion on thorium hydrocarbyls complexes [223]

In a similar manner, uranium complexes of the type (η5 -C5 H4 R)3 UR′ (where R′ = CH3 , C2 H5 , i-C3 H7 , n-C4 H9 , t-C4 H9 , N(C2 H5 )2 and even P(C6 H5 )2 and NCBH3 ) [224, 225] also undergo CO insertion reactions. Mechanistic studies [225] showed that the insertion reaction appears first-order under the conditions of excess CO. The rate of insertion depends on the steric factors of the cyclopentadienyl ring and with different substituent follows the order H > Me > i-Pr > t-Bu. Interestingly the rate also depends on the identity of the alkyl ligand with an unusual order R′ = n-Bu > t-Bu > Me > i-Pr. The resulting η2 -acyl product was not stable and rearranged to yield alkylbenzenes C6 H4 RR′ , suggested to arise from ring enlargement of the cyclopentadienyl ligand by incorporation of the CR′ fragment. Likewise, CO2 reacts with (Cpφ )3 UH (where Cpφ = η5 -C5 H4 SiMe3 ) [226] to afford the formate derivative (Cpφ )3 UOCHO, which further reacted with the starting uranium hydride to give the dioxymethylene complex (Cpφ )3 UOCH2 OU(Cpφ )3 :

Metallocene Organoactinide Complexes

49

Isoelectronic isocyanide ligands also undergo insertion into uranium– carbon or uranium–nitrogen bonds [227, 228] to yield η2 -iminoalkyl (11) and η2 -iminocarbamoyl (12) adducts:

Cramer et al. synthesized an unusual class of complexes of the type (η5 -C5 H5 )3 UY, which contains M–carbon/nitrogen multiple bond character [229–234]. They were also the pioneers in synthesizing the first actinide complexes containing phosphorus ligands (Scheme 17).

Scheme 17 Synthesis of various uranium phosphorus complexes [236, 238]

Among these, the complex [{µ-(CH)(CH2 )P(C6 H5 )2 }U(C5 H5 )2 ]2 [235, 236] possesses an unusual coordination number for U(IV) and exhibits a unique mode of ylide bonding in which the ligand chelate as well as bridge between two metal centers (Fig. 19). The geometry about the each uranium A atom is approximately tetrahedral with a U-U bond distance of 3.810(2) ˚ A). The (Table 10), which is at the limit of van der Waals interactions (3.8 ˚ U–C bond lengths 2.46(1) and 2.53(2) ˚ A in the U–C–U bridge were within the range of U–C σ-bonds found in several tris(cyclopentadienyl)uranium alkyl A), A), Cp3 UC4 H9 (2.426(23) ˚ complexes like Cp3 U(CH2 )2 CCH3 [219] (2.48 ˚ ˚ and Cp3 UCH2 (p-CH3 C6 H4 ) (2.541 A) [117] (Table 10). Another interesting complex [(η5 -C5 H5 )2 Th(η5 , η1 -C5 H4 )]2 was reported which contains an unusual µ-carbon bridge [237], where the cyclopentadienyl group was bonded in the pentahapto fashion towards one of the thorium atom, while to the other it is bonded through a σ-bond.

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Fig. 19 Crystal structure of [{µ-(CH)(CH2 )P(C6 H5 )2 }U(C5 H5 )2 ]2 [236]. Reprinted with permission from [236];  (1980) American Chemical Society

Some other interesting uranium(IV) phosphoylide complexes, Cp3 UCHPMe2 (Ph), Cp3 UCHPMe(Ph)2 , CpU[(CH2 )2 P(Ph)2 ]3 , and CpU[(CH2 )2 P(Me)(Ph)]3 , were synthesized [238] by the metathesis reaction of Cp3 UCl with LiR (where R = –CH2 CH2 P(Me)(Ph), –CH2 CH2 P(Ph)2 ) (Scheme 17). The molecular structure of the complex Cp3 UCHPMe2 (Ph) reveals that the uranium is bonded in tetrahedral fashion to three cyclopentadienyl ligands and fourth to the ligand “R” [229, 231]. The U–C bond was found to be the shortest in all these kinds of complexes (Table 10), which suggests multiple bond character. This was explained via the following two resonance forms (C and D) of the ligand:

In resonance form D, the carbon atom carried two pairs of electrons and has a double negative charge. Upon coordination to the metal center Cp3 U+ the resonance structures E–G may be written:

Metallocene Organoactinide Complexes

51

Working on this series of compounds, Cramer et al. reported [234] a complex which contains an uranium–nitrogen multiple bond, i.e., an (imido)uranium complex (C5 H5 )3 UNC(CH3 )CHP(C6 H5 )2 CH3 (Eq. 42):

(42)

A), which The U–N bond distance was found to be very short (2.06(1) ˚ suggests the existence of a multiple bond character. On the other hand, the bond angles around the C-atoms attach to the N or P are consistent with sp2 hybridized state. Therefore, it was suggested that the molecule (C5 H5 )3 UNC(CH3 )CHP(C6 H5 )2 CH3 might have the resonating structures as shown:

Their combination implies a highly delocalized π system and an uranium– nitrogen bond order between two and three. Insertion reactions were extensively investigated in the complexes containing metal–ligand multiple bonds [49, 230, 233, 234, 239, 240]. The complexes Cp3 U = CHP(CH3 )(C6 H5 )R1 (13) are found to undergo CO insertions to give the complexes of the type Cp3 U(η2 -OCCH)P(CH3 )(C6 H5 )R1 (14), where R1 is either CH3 (14a) or C6 H5 (14b), [230]. The complex Cp3 U(η2 -OCCH)P(CH3 ) (C6 H5 )2 was characterized by single crystal X-ray diffraction study and the U–C(1), U–O and U–Cp(av) distances were found to be 2.37, 2.27, and A, respectively. The insertion of Ph–N=C=O to complex 13 gives Cp3 U 2.81 ˚ [(NPh)(O)CCHP(Me)(Ph)R] (15a, R = Me; 15b, R = Ph) [49]. The complex 15a was structurally characterized and found to have a four-membered chelate ring. The ligand Ph–N=C=O coordinated to the pyramidal Cp3 U+ unit through its N- and O-coordination sites in such a way that minimized the steric interaction [49]. The U–O and U–N bond distances were found to be A, respectively, and are consistent with a single bond. The 2.34(1) and 2.45(1) ˚ O–C and N–C bond lengths lie between the single and double bonds. In view of these, the bonding in 15 was best described with the help of the following resonating structures:

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Bimetallic complexes of the type Cpn (OC)m MC(OUCp3 )=CHPMePhR1 (M = Mn, n = 1, m = 2 (16); M = W, n = 0, m = 5 (17); M = Co, n = m = 1 (18): R = Me (18a), Ph (18b)) were synthesized by the insersion of terminal CO from the starting complexes like CpMn(CO)3 , W(CO)6 and CpCo(CO)2 to Cp3 U=CHP(Ph)(R)Me respectively [233, 239, 241]. On the basis of the crystal data, the structure of the complex Cp(CO)2 MnC(OUCp3 )=CHP(Ph)Me was best interpreted with the help of the following resonating structures:

The complexes (OC)5 WC(OUCp3 )=CHPMe(R)Ph undergo isomerization at 90 ◦ C to form Cp3 UOCH=CHPPh(R)CH2 W(CO)6 (19), R = Me (19a), Ph (19b) [239]. Some important bond lengths around the U metal center are given in Table 11. The reaction between Cp3 U=CHP(CH3 )(C6 H5 )2 and HN(C6 H5 )2 produces Cp3 UN(C6 H5 )2 in good yield [240]. The X-ray structure of Cp3 UN A, which indicates that the (C6 H5 )2 shows the U–N bond distance of 2.29 (1) ˚ U–N bond order is close to two. The reaction of Cp3 AnCl with LiNPPh3 produces Cp3 AnNPPh3 (An = U or Th). The molecular structure of Cp3 UNPPh3 was determined by single crystal X-ray crystallography [242] and the geometry around the U and P atoms was found to have the usual tetrahedron orientation. The average U–C(Cp) bond distance, 2.78(2) ˚ A, is in the range reported for other Cp3 U–X type of complexes, but the U–N and U–P bond lengths are significantly shorter compared to the transition metal–phosphine imine complexes. Based on these structural parameters, the bonding in these phosphine imide complexes could only be described with the help of the following resonating structures:

But, when electron delocalization does not occur within the R group of the N atom, the bonding consists of less resonating structures:

Metallocene Organoactinide Complexes

53

Table 11 Some selected bond lengths of bimetallic complexes of uranium(IV) Complex

Bond length (˚ A) U–Cp(I) U–Cp(II) U–Cp(III) U–Y(1)

Refs.

Cp(OC)2 MnC(OUCp3 )=CHPMe2 Ph (OC)5 WC(OUCp3 )=CHPMePh2 Cp3 UOCH=CHPPh2 CH2 W(CO)6 Cp(OC)CoC(OUCp3 )=CHPMe2 Ph

2.504 2.472 2.465 2.498

[233] [239] [239] [241]

2.513 2.516 2.477 2.496

2.495 2.482 2.470 2.489

U–O 2.13(2) U–O 2.15(2) U–O 2.207(9) U–O 2.08(2)

After the successful study of many An(IV) tris cyclopentedienyl complexes, chemists looked for some kind of organoactinide complexes that would have less steric hindrance. In view of this, synthesis of biscyclopentadienyl complexes of the type (η5 -C5 H5 )2 AnX2 was explored, although these were difficult to synthesized due to ligand distribution to yield mono- and triscyclopentadienyl species [243]. Alternative approaches to generate complexes of this formula have generally involved introduction of the cyclopentadienyl ligands in the presence of other ligands that inhibit redistribution, as in Eqs. 43–45 [244–247]:

(43) (44)

(45) The complex [Cp2 U{N(C2 H5 )2 }2 ] reacts with ligands that have protons of more acidic nature than that of diethylamine [244], like toluene-3,4-dithiol, omercaptophenol, catechol, and 1,2-ethanediol (Eq. 46): (46) In 1978, Manriquez et al. reported [248] new bis(pentamethylcyc1opentadienyl) derivatives of thorium and uranium, which may be considered as the most chemically reactive and versatile organoactinides prepared up to that time. Following this, a number of communications came out with the

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M. Sharma · M.S. Eisen

synthesis of successfully stabilizing bis(cyclopentadienyl) complexes involving the use of peralkylated derivatives (C5 Me5 [249]; C5 Me4 Et [250]). The pentamethylcyclopentadienyl ligand is one of the most widely used ligands in organoactinide chemistry because the incorporation of this ligand substantially increases the stability, solubility, and crystallinity of the obtained compounds. Initial synthetic routes involved alkylation of the metal tetrahalides by Grignard or tin (Eqs. 47–49) reagents:

(47)

(48)

(49) The molecular structure of many of these dihalide complexes, (η5 -C5 Me5 )2 UCl2 [251], (η5 -C5 Me5 )2 ThX2 (X = Cl, Br, I) [251–253] shows monomeric structures with a pseudotetrahedral, “bent metallocene” geometry having C2v symmetry group. The dichloride derivative can easily be alkylated with variety of alkyl- and aryllithium reagents to form dialkyl and diaryl complexes (Eq. 50) [249]:

(50) Recently, Barnea et al. reported the synthesis of Cp∗ 2 UMe2 by the reaction of Cp∗ 2 UCl2 with methyl lithium in presence of lithium bromide [254]. The crystal structure of Cp∗ 2 UMe2 (Fig. 20) was also found to have a similar type of geometry to that of Cp∗ 2 AnX2 . Apart from the per-substituted cyclopentadienyl ligand, other ligands like [1,3-(Me3 Si)2 C5 H3 ] and [1,3-(Me3 C)2 C5 H3 ] have also drawn attention to the field of organometallic chemistry. Metal complexes containing these ligands

Metallocene Organoactinide Complexes

55

Fig. 20 Crystal structure of {[η5 -C5 Me5 ]2 UMe2 } [254]. Reprinted with permission from [254];  (2004) American Chemical Society

have been prepared by reacting metal tetrahalides with the cyclopentadienyl lithium reagents (Eq. 51) [255]:

(51) Luken et al. [122] reported successful synthesis of uranium metallocenes complexes by using magnesocenes as reagents (Scheme 18). The molecular structures of the complexes [L2 UX2 ] (where L = η5 -1,3-(R2 C5 H3 )2 UX2 and R = SiMe3 , X = F, Cl, Br; or R = t-Bu, X = F, Cl) characterized by single crystal X-ray crystallography and found isostructural with [η5 -1,3-(Me3 Si)2 C5 H3 ]2 ThCl2 . Except for the dimeric fluoride complex,

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M. Sharma · M.S. Eisen

Scheme 18 Synthesis of various uranium metallocene complexes [122]

{[η5 -1,3-(Me3 Si)2 C5 H3 ]2 UF(µ–F)}2 (Fig. 21), all the other complexes exists as monomers in solid state [122]. But in solution {[η5 -1,3-(Me3 Si)2 C5 H3 ]2 UF(µ-F)}2 also presents in a monomer–dimer equilibrium form. The complexes Cp2 UX2 have an idealized C2v structure when X is F, Cl, or Br and a C2 structure when X is I or Me; the conformations of the substituted Cp ligands are directly related to the radii of the X ligands. Some important bond lengths are given in Table 12. Although in these complexes the bulky cyclopentadienyl ligands provide kinetic stability, in a limited number of cases base adduct complexes like (η5 -C5 Me5 )2 UCl2 (pz) (pz = pyrazole) [256], [η5 -1,3-(Me3 Si)2 C5 H3 ]2 ThCl2 (dmpe) [19] have been generated. The complex (η5 -C5 Me5 )2 U(OTf)2 (H2 O) (OTf = trifluoromethylsulfonate) was isolated in low yield from the reaction of (η5 -C5 Me5 )2 UMe2 with triflic acid [196]. In compounds of the for-

Table 12 Some important bond lengths of bis(substituted cyclopentadinyl)actinide(IV) complexes of the type Cp2 AnX2 Complex

(η5 -C5 Me5 )2 ThCl2 (η5 -C5 Me5 )2 ThBr2 (1,3-(Me3 Si)2 C5 H3 )2 UCl2 (1,3-(Me3 Si)2 C5 H3 )2 ThCl2 (1,3-(Me3 Si)2 C5 H3 )2 UBr2 (1,3-(Me3 Si)2 C5 H3 )2 UI2 (1,3-(Me3 Si)2 C5 H3 )2 U(BH4 )2 (1,3-(Me3 Si)2 C5 H3 )2 UCl2 (1,3-(Me3 C)2 C5 H3 )2 UCl2 (1,3-(Me3 C)2 C5 H3 )2 UF2 (1,3-(Me3 C)2 C5 H3 )2 UMe2 ∗

centroid of the Cp ring

Bond distance (˚ A) M–Cp average

M–X

2.78(2) 2.51 ∗ 2.72(1) 2.78(1) 2.71(2) 2.70(3) and 2.72(3) 2.72(2) 2.49 ∗ 2.49 ∗ 2.46 ∗ 2.44 ∗

2.600(5) 2.800(2) 2.579(2) 2.632(7) 2.734(1) 2.953(2) and 2.954(2) 2.56(1) 2.573(1) 2.577(4) 2.086(2) 2.42(2)

Refs.

[126] [252] [255] [255] [255] [255] [255] [123] [123] [123] [123]

Metallocene Organoactinide Complexes

57

Fig. 21 Crystal structure of {[η5 -1,3-(Me3 Si)2 C5 H3 ]2 UF(µ-F)}2 [122]. Reprinted with permission from [122];  (1999) American Chemical Society

mula (η5 -C5 Me5 )2 UX2 (L) (L = neutral ligand), the coordinated base generally occupies the central position in the equatorial wedge. The introduction of the permethyl substituted cyclopentadienyl ligand into the coordination sphere of actinides afforded complexes with advantageous solubility, crystallizability, thermal stability, and resistance to ligand redistribution. However, abundant structural data indicate that despite these advantages the metal center suffers from a high steric congestion that may decrease

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the reactivity relative to the known or hypothetical bis(cyclopentadieny1) analogs. In an effort to “open” the actinide coordination sphere while preserving the frontier orbitals and other advantages of Cp∗ , a totally new approach was made by linking two substituted cyclopentadienyl rings to afford a chelating bis-(permethylcyclopentadienyl) ligand [Me2 Si(Me4 C5 )2 ]2– , [R2 Si(Me4 C5 )(C5 H5 )]2– [257, 258] (Scheme 19).

Scheme 19 Synthesis of chelating bis(cyclopentadienyl) ligand [257]

With these new sets of ligands a number of complexes have been reported of the type Me2 SiCp′′′ 2 AnCl2 · xLiCl · y(sol) (where Cp′′′ = (Me4 C5 ); An = Th, U [257, 259, 260]). The complex of the type [(Cp′′′ )2 (µ-SiMe2 )] U(µ–Cl)4 [Li(tmeda)]2 was obtained when (Cp′′′ )2 (µ-SiMe2 )UCl2 · 2LiCl2 · 4Et2 O was recrystallized from toluene and N,N,N ′ ,N ′ -tetramethylethylenediamine(tmeda). A typical bent structure was observed for the complex [(Cp′′′ )2 (µ–SiMe2 )] U(µ-Cl)4 [Li(tmeda)]2 . The ring centroid–U–ring centroid angle (114.1◦ ) is considerably smaller than that observed in nonchelated bis(cyclopentadienyl) uranium complexes (133–138◦ ) and is comparable to the angle determined for the thorium dialkyl complex [Me2 Si(C5 Me4 )2 ]Th(CH2 Si(CH3 )3 )2 (118.4◦ ) [257]. The contraction of the centroid–metal–centroid angle clearly indicates widening of the equatorial face of these types of complexes, which enhances room in its coordination sphere and thus further facilitates its reactivity. The uranium is coordinated to four bridging chloride ligands. Two of the uranium chloride bond distances U(1)–Cl(1) A are longer than the U(1)–Cl(3) 2.760(3) and 2.885(3), U(1)–Cl(2) 2.853(3) ˚ ˚ U(1)–Cl(4) 2.746(3) A. These complexes can easily be alkylated by lithium alkyl or Grignard reagents [257]. Wang et al. reported a similar type oxygen and chloride bridge complex [{[η5 -(C5 Me4 )2 SiMe2 ]UCl}2 (µ-O)(µ-Cl)·Li·1/ 2DME]2 (Fig. 22), which was found to be very reactive and undergo facile alkylation to yield oxide-bridge dibutyl uranium complex [{[η5 -(C5 Me4 )2 SiMe2 ]U(Bu)}2 (µ-O)] (Scheme 20) [260].

Metallocene Organoactinide Complexes

59

Scheme 20 Synthesis of O-bridged chelating bis(cyclopentadienyl) uranium complex [260]

Fig. 22 Crystal Structure of [{[η5 -(C5 Me4 )2 SiMe2 ]UCl}2 (µ-O)(µ-Cl)·Li·1/2DME]2 [260]. Reprinted with permission from [260];  (2000) American Chemical Society

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Following the establishment of this new series of complexes, their chemistry was well studied and a number of dialkyl derivatives, [(µ-SiMe2 ) (η5 -C5 Me4 )2 ]ThR2 (where R = CH2 SiMe3 , CH2 CMe3 , C6 H5 , n-C4 H9 , and CH2 C6 H5 ) have also been reported [257, 261]. The dialkyl complexes undergo rapid hydrogenolysis under H2 to yield a light-sensitive dihydride complex {[(µ-SiMe2 )(η5 -C5 Me4 )2 ]ThH2 }2 . IR spectroscopy and structural data (a short A) is in good agreement with the formulation of Th–Th distance of 3.632(2) ˚ the compound having two bridging hydride ligands. Looking at the aspect that the “tying” back of the Cp ligands provides many facilities to the metal center, other sets of ligands have been explored in which the two Cp rings were back-bonded with some bridging groups, like pyridine [262], ROR [263] etc. Paolucci et al. [262] reported a mononuclear bridged bipyridine Cp2 UCl2 derivative, µ-{2, 6-CH2 C5 H3 NCH2 }(η5 -C5 H4 )2 UCl2 according to Eq. 52:

(52) According to the single-crystal X-ray study of this complex, the U–Nav A, which clearly indicates a strong U–N interaction bond distance was 2.62(1) ˚ along with two Cl ligands (U–Cl 2.615(3) and 2.636(3) ˚ A). A plethora of bis(pentamethylcyclopentadienyl)uranium complexes have been synthesized and many of them were characterized crystallographically. Some of such complexes and their important U–Cpav and U–X bond distances are given in Table 13. In this respect, Th does not lag behind and was found to be the same in reactivity and to form almost similar types of derivatives [249, 264, 265]. These complexes were produced either by metathesis or by protonation reactions. A number of mixed alkyl–halide complexes were prepared, mainly by reaction of (η5 -C5 Me5 )2 AnCl2 with one equivalent of alkylating agent, although the methyl–chloride complex is best prepared by redistribution from the dichloride complex Cp∗ 2 AnCl2 and dimethyl complexes Cp∗ 2 AnMe2 . Straub et al. reported the synthesis of Cp∗ 2 Th(Cl)(Me) by the reaction of an equimolar mixture of Cp∗ 2 ThCl2 and Cp∗ 2 ThMe2 in toluene [266]. The complex was characterized crystallographically (Fig. 23) and found to have normally disposed organoactinide metallocene with a ring centroid–metal–ring centroid angle of 135(2)◦ , metal to ring centroid disA). A, and Th–CH3 /Cl average distance of (∼ 2.67(8) ˚ tance of 2.56(9) ˚ Alkyl complexes of the type (Cp∗ )2 AnR2 are very reactive and undergo benzonitrile insertion reaction into the actinide–carbon bonds to afford bis(ketimide) complexes (Cp∗ )2 An[N=C(Ph)(R)]2 (where An = Th, U; R = CH3 , CH2 Ph, Ph) [267–270]. More recently, it has been reported that treatment of (Cp∗ )2 Th(CH3 )2 with excess 4-fluorobenzonitrile yielded

Metallocene Organoactinide Complexes

61

Fig. 23 Crystal structure of Cp∗ 2 Th(Cl)(Me) [266]. Reprinted with permission from [266];  (2001) American Chemical Society

bis(ketimide) complex (Cp∗ )2 Th[N=C(CH3 )(4-F–C6 H4 )]2 as the major product along with eight-membered thorium(IV) tetraazamacrocycle complex [271] as shown in Eq. 53:

(53) Reaction of (η5 -C5 Me5 )2 ThCl2 with the bulkyl silyl salt (THF)3 Li [Si(SiMe3 )3 ] yields an unstable complex (η5 -C5 Me5 )2 Th(Cl)[Si(SiMe3 )3 ] that could be trapped by the reaction with two equivalents of carbon monoxide to produce a ketene complex (η5 -C5 Me5 )2 Th(Cl)[O–C(=C=O)Si(SiMe3 )3 ]. In contrast, (η5 -C5 Me5 )2 ThCl{Si–(t-Bu)Ph2 } could be isolated and its reaction with CO gave a similar silylthoroxyketene compound, and in this case the

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Table 13 Some important bond lengths of various U(IV) complexes containing η5 -C5 Me5 ligand Complex

Bond distance (˚ A) U–Cp U–X average

Refs.

[Cp∗ 2 U]2 [η2 -CO(NMe2 )]2

2.78(2) 2.342(7) O 2.405(8) C 2.402(9) C 2.74(3) 2.743(1) P 2.77(2) 2.658(4) Cl 2.43(1) N 2.78(3) 2.62(1) C 2.58(1) C 2.80(3) 2.62(3) C 2.54(3) C 2.72(2) 2.74(2) 2.790(12) 2.951(15) C 2.71(6) 3.094(5) I 3.115(5) I 2.943(5) I 2.779(15) S 2.77(2) 2.051(14) N 3.19(4) Li 2.77(6) 2.646 Cl 2.438 N 2.78(3) 2.79(2) 2.370(5) O 2.77(2) 2.60(1) C 2.58(1) C 2.73(2) 2.79(3) 2.477(2) S 2.75(3) 2.885(4) S2 2.812(5) S3

2.370(5) O

[273]

2.046(14) O

[281]

2.730(4) Cl

[313]

2.658(2) Cl

[316]

2.680(8) Cl

[316]

2.583(6) 2.58(3) B 1.952(12) N

[251] [248] [278]

3.240(5) I

[157]

2.690(5) Cl

[278]

2.693 Cl

[314]

2.267(6) 2.2562(3) P

[307] [280]

2.22(1) N

[312]

2.639(3) S 2.744(2) S

[284] [141]

2.643(4) S1

[284]

[Cp∗ 2 U(OMe)]2 PH Cp∗ 2 UCl2 (HNPPh3 ) Cp∗ 2 UCl[(CH2 )(CH2 )P(Ph)(Me)] Cp∗ 2 UCl[(CH2 )(CH2 )P(Ph)2 ] (η5 -C5 Me5 )2 UCl2 Cp∗ 2 U(BH4 )2 Cp∗ 2 U(N–2,4,6-t-Bu3 C6 H2 ) Cp∗ 2 U3 (µ3 -I)(µ3 -S)(µ2 -I3 )I3

[Li(tmed)][Cp∗ 2 U(NC6 H5 )Cl] Cp∗ 2 UCl2 (HNSPh2 ) Cp∗ 2 U(NH(C6 H3 Me2 -2,6))2 [Cp∗ 2 U(P–2,4,6-t-Bu3 C6 H2 )](OPMe) Cp∗ 2 U(NMe2 )(CN–t-Bu)2 [BPh4 ] Cp∗ 2 U(SMe)2 [Na-(18-crown-6)(THF)2 ][Cp∗ 2 U(SBu-t)(S)] Cp∗ 2 U(S–t-Bu)(S2 CBu-t)

Metallocene Organoactinide Complexes

63

Table 13 (continued) Complex

Bond distance (˚ A) U–Cp U–X average

Refs.

Cp∗ 2 UCl(η2 -t-BuNSPh)

2.76(2) 2.20(2) N 2.825(8) S 2.80(2) 2.309(6) N 2.840(4) S 2.79(2) 1.85(1) S 2.44(1) C 2.81 2.777(1) S 2.77(3) 2.185(5) N2 2.73(2) 2.80(5) 2.637(9) C 2.74 2.125(13) O 2.78(2) 2.47(2) N 2.746(4) Cl 2.71(2) 2.393(12) C 2.73(2) b 2.650(3) S2 2.75(2) 2.78(4) 2.090(8) N c 2.467(8)– 2.494(7) N d 2.74(5) 2.942(3) I 3.092(2) I 2.77(2) 2.561(13) C 2.505(14) C 2.77 2.205 N 2.77(8) 2.135(8) N

[310]

Cp∗ 2 UBr(η2 -t-BuNSPh) [Na-(18-crown-6)(THF)2 ][Cp∗ 2 U(SMe)(SCH2 )] [Na-(18-crown-6)(THF)2 ][Cp∗ 2 U(SPr-i)2 ] Cp∗ 2 U(N=CPh2 )2 Cp∗ 2 UMe2 Cp∗ 2 U(O)[C(NMeCMe)2 ] [Cp∗ 2 U]2 (µ-O) Cp∗ 2 UCl(OH)(HNSPh2 ) Cp∗ 2 UMe(THF)[MeBPh3 ] Cp∗ 2 U(dddt)a Cp∗ 2 U(C4 Ph4 ) [Cp∗ 2 U(µ-N)U(µ-N3 )Cp∗ 2 ]4

Cp∗ 2 UI2 (NCPh) Cp∗ 2 U(CH2 Ph)[η2 -(O, C)-ONC5 H4 ] Cp∗ 2 U[N=C(CH2 C6 H5 )(tpy)]2 Cp∗ 2 U[N=C(CH2 C6 H5 )(tpy)]2 YbCp∗ 2

2.628(7) Cl

2.794(12) Br [310]

2.613(3) S

[140]

2.791(1) S

[140]

2.179(6) N1

[267]

2.409(5) 1.917(6) O

[254] [107]

2.094(14) O

[315]

2.117(9) O

[315]

2.419(8) O

[174]

2.629(3) S1

[153]

2.395(2) C 2.055(8)–

[104] [308]

2.53(1) N

[309]

2.361(9) O

[311]

2.205 N

[306]

2.054(8) N

[306]

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Table 13 (continued) Complex

Bond distance (˚ A) U–Cp U–X average

[Cp∗ 2 U(NCMe)5 ][BPh4 ]

2.80(1)

[Cp∗ 2 U(NCMe)5 ]I2

2.80(1)

2.537 2.570 2.529 2.576 2.521 2.556 2.547 2.548 2.535 2.551

Ref.

N N N N N N N N N N

[317]

[317]

a

5,6-Dihydro-1,4-dithiin-2,3-dithiolate Centrid c Nitride d Azide b

transient η2 -acyl complex (η5 -C5 Me5 )2 ThCl[η2 -CO{Si–(t-Bu)Ph2 }] could be detected [272]. Metathesis and protonation reactions are found to be very important reaction as they produce a variety of derivatives of various bis-substituted pentadienyl complexes [249, 256, 273]; even many metallocene phosphide complexes have been generated by this route [274, 275]. Polysulfides are also a very interesting class of chelating ligands. The complex Cp∗ 2 ThCl2 undergoes metathesis reaction with Li2 S5 to form Cp∗ 2 ThS5 [276]. In a similar manner, the complex (C5 Me4 Et)2 ThS5 was also synthesized and characterized by NMR spectroscopy. It has been proposed that the complexes (C5 Me4 Et)2 ThS5 consists a fluxional twist-boat ThS5 ring conformation [276] and the energy barrier to the interconverting isomers was ca. 57.4 kJ mol–1 :

A series of bis(pentamethylcyclopentadienyl)uranium and thorium complexes containing bis(trimethylsilyl)phosphide ligand, Cp∗ 2 An(X)[P(SiMe3 )2 ], have been synthesized, where X = C1 (An = U, Th) or CH3 (An = U,

Metallocene Organoactinide Complexes

65

Th). Thermal decomposition of the complexes (Cp∗ 2 )2 AnMe[P(SiMe3 )2 ] results in the formation of the P–C bridging complexes (Cp∗ 2 )2 AnMe [η2 -P(SiMe3 )SiCH3 CH2 ] (Eq. 54):

(54) It has been observed that the trivalent organouranium complexes undergo one- or two-electron transformations into organic molecules. To extend this concept to the actinide(IV) complexes, Brennan et al. synthesized complexes of the type [(RC5 H4 )3 U]2 [µ-η1 ,η2 -CS2 ] [72], where R is Me or SiMe3 , with the aim that these may also act as a tight-ion-pair complex of CS2 2– . Similar type of bridging complexes (MeC5 H4 )4 U2 (µ-NR)2 [130, 277] were also synthesized and structurally characterized. The crystal structures of (MeC5 H4 )4 U2 (µ-NR)2 (where R = Ph and SiMe3 ) revealed that the NPh group asymmetrically bridges the two uranium fragments, whereas the NSiMe3 group symmetrically bridges the two uranium fragments. The bridgA in the NPh complex (Fig. 24) ing U–N distances are 2.156(8) and 2.315(8)˚ and 2.217(4) and 2.230(4) ˚ A, in the NSiMe3 complex. Organoimido ligands have proven to be of great interest to synthetic chemists, but they are still dominating the d-element complexes. Burns and coworkers tried to explore the chemistry of this potential ligand towards

Fig. 24 Crystal structure of (MeC5 H4 )4 U2 (µ-NPh)2 [277]. Reprinted with permission from [277];  (1988) American Chemical Society

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f -elements by synthesizing metallocene complexes (η5 -C5 R5 )2 An(=NR′ ) [278]. They reported the synthesis of a number of monoimido derivatives of U(IV) by both metathesis and direct protonation routes [278]. The base free neutral monoimido complex (η5 -C5 Me5 )2 U(=N–2,4,6-(t-Bu3 )C6 H2 ) could have been prepared by α-elimination reactions as shown in Scheme 21.

Scheme 21 Synthesis of various U(IV) organoimido complexes from (Cp∗ )2 UMe2 [278]

The molecular structure of the complex (η5 -C5 Me5 )2 U{=N–2,4,6(t-Bu3 )C6 H2 } displays considerable asymmetry in the conformation of the two ortho tert-butyl groups with respect to their orientation toward the uranium metal center. As expected from the steric point of view, one of the tert-butyl groups oriented in space in such a way that its methyl molecules can stay as far as away from the uranium atom. In contrast, one of the methyl molecules of the other tert-butyl group pointed direct1y toward A and with an C–U–N the uranium metal center at a distance of 2.951(15) ˚ A) sugangle of 66.2(5)◦ . The extremely short U–N bond length (1.952(12) ˚ gests a relatively high formal bond order where one or both nitrogen lone pairs are involved in bonding to the uranium atom. The steric bulk of the aryl group is important in stabilizing a base free organoimido complex; the smaller (η5 -C5 Me5 )2 U{=N–2,6-(i-Pr)2 C6 H3 } is best isolated as the THF adduct [278], and the parent phenylimido has only been isolated as a uranate salt, [Li(tmeda)][(η5 -C5 Me5 )2 U(=NC6 H5 )Cl] [278]. The sigA and the U–N–C bond nificantly short U–N bond distance of 2.51(14) ˚ angle of 159.8(13)◦ (Table 13) indicate the polarization of the lone pair of electrons on the nitrogen towards the uranium center. The complex (η5 -C5 Me5 )2 U(=N–2,6-Me2 C6 H3 ) has also been reported. The organoimido

Metallocene Organoactinide Complexes

67

Fig. 25 Crystal structure of (Cp∗ )2 Th(=N-2, 6-Me2 C6 H3 ) [279]. Reprinted with permission from [279];  (1996) American Chemical Society

complexes of the type Cp∗ 2 An(=NR), (where An = U and Th) have been found as a reactive species in the catalytic cycle of hydroamination of terminal alkynes [266, 279]. The monoimido thorium complex (Cp∗ )2 Th(=N–2,6-Me2 C6 H3 ) has been synthesized by the reaction of (Cp∗ )2 ThMe2 with 2,6-dimethylaniline in THF and was structurally characterized as a mono-THF adduct (Fig. 25) [279]. By applying almost the same synthetic method, the phosphinidine analog of uranium has been synthesized (Eq. 55) [280]:

(55) In the complex (Cp∗ )2 U{P–2,4,6-(t-Bu)3 C6 H2 }(OPMe3 ), the uranium atom lies at the center of a tetrahedral with the U–O and U–P bond A, respectively (Table 13). Interestingly, distance of 2.370(5) and 2.562(3) ˚ when the bis-cyclopentadienyl ligand was replaced by bridging ansa-ligand {(R4 C5 )2 (µ-SiMe3 )} (where R = CH3 or H) the reaction of {(R4 C5 )2 (µ-SiMe3 )} UMe2 with H2 –P–2,4,6-(t-Bu)3 C6 H2 produced the dimeric complexes {(R4 C5 )2

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(µ-SiMe2 )U(µ-P–2,4,6-t-Bu3 C6 H2 )}2 , (where R = Me or H) (Eq. 56):

(56) The hydride bridge actinide complex, [(η5 -C5 Me5 )2 UH2 ]2 , reacts with P(OMe)3 to generate a bridging phosphinide complex [(η5 -C5 Me5 )2 U(OMe)]2 (µ-PH) by P–O cleavage with sacrificial formation of (η5 -C5 Me5 )2 U(OMe)2 [281]. The molecular structure of [(η5 -C5 Me5 )2 U(OMe)]2 (µ-PH) has a C2 symmetry, with the µ-PH2– ligand lying on a crystallographic twofold axis. The coordination geometry about each uranium ion is of the typical pseudotetrahedral. With the help of the metathesis reaction of Cp∗ 2 ThC12 with LiPR2 , the first diorganophosphido actinide complexes Cp∗ 2 Th(PR2 ) (where R = Ph, Cy, Et) were prepared [274]. The molecule Cp∗ 2 Th(Ph2 ) exhibits a pseudotetrahedral geometry about the thorium atom with the pentamethylcyclopentadienyl ligands and two diphenylphosphido groups occupying the four coordination sites. The angles about the phosphorus atoms are far from the tetrahedral, and A). there is no evidence for significant Th–P multiple bonding (Th–Pav 2.87 (2) ˚ The cothermolysis of the butadiene complex (η5 -1,3-t-Bu2 C5 H3 )2 Th(η4 -C4 H6 ) with P4 or As4 gives the binuclear Th complex [(η5 -1,3-t-Bu2 C5 H3 )2 Th]2 (µ,η3 ,η3 -E6 ) (where E = P, As) [282, 283]. When the reaction of P4 was carried out in presence of MgCl2 only the complex [(η5 -1,3-t-Bu2 C5 H3 )2 Th](µ,η3 -P3 )[Th(Cl)(η5 -1,3-t-Bu2 C5 H3 )2 ] was formed (Scheme 22). Although the monomeric complexes of actinides with N, P or O donor ligands are stable and could be synthesized without much problem, mono- and dithiol complexes are very difficult to synthesized because they tend to support a monomer–dimer equilibrium [244]. Only lately have a few complexes of bis(pentamethylcyclopentadienyl) metallocene dithiolates, (η5 -C5 Me5 )2 Th(SPr)2 [202] and (η5 -C5 Me5 )2 U(SR)2 (where R = Me, i-Pr, t-Bu, Ph) [284] appeared. The C–S bond cleavage in the complexes [(Cp∗ )2 U(SR)2 ] (where R = i-Pr, and t-Bu) has been studied and it was found that the complex (η5 -C5 Me5 )2 U(SBu-t)2 undergoes reduction by Na–Hg with cleavage of a C–S bond [141]. The product was isolated with an 18-crown-6-ether and proved to be a complex with a terminal sulfido ligand bound to the sodium coun-

Metallocene Organoactinide Complexes

69

Scheme 22 Reactivity of P4 and As4 towards thorium butadiene complex [282, 283]

terion. The complex [Na(18-crown-6-ether)][(η5 -C5 Me5 )2 U(SBu-t)(S)] posA than the typical sesses a significantly shorter U–S bond distance of 2.462(2) ˚ U–SR bond ca. 2.64 ˚ A). Recently, the diene compounds have emerged into the field of organometallic chemistry as a successful ligand. However, the existence of diene complexes of 5f -elements was in a big shadow. Marks and coworkers [264] took the challenge to synthesize the actinide cis-2-butene-1,4-diyl complexes Cp∗ 2 U(η4 -C4 H6 ), Cp∗ 2 Th(η4 -C4 H6 ), and Cp∗ 2 Th(η4 -CH2 CMeCMeCH2 ) from their corresponding halides Cp∗ 2 MC12 (where M = U, Th) and the appropriate (THF)2 Mg(CH2 CRCRCH2 ) reagent. The molecular structure of Cp∗ 2 Th (η4 -C4 H6 ) has been determined by single-crystal X-ray diffraction and was found to consist of a “bent sandwich” (Cp∗ )2 Th fragment coordinated to an s-cis-η4 -butadiene ligand (Fig. 26). The crystal structure supports the η4 -hapticity of the butadiene ligand. The average Th–C distance to the terminal carbon atoms of the butadiene A) is only slightly smaller than that to the internal carbon ligand (2.57(3) ˚ A), and are comparable to those found in other thorium alkyl atoms (2.74(2) ˚ complexes. The C(1)–C(2) and C(3)–C(4) average distances (average of four A, which is compared to the independent molecules in the unit cell) is 1.46(5) ˚ ˚ average C(2)–C(3) distance of 1.44(3) A. The actinide butadiene complexes undergo inversion of the metallacyclopentene ring, which is rapid on the NMR time scale at higher temperatures (Scheme 23). The measured energy barrier ∆G* (Tc , K) is 17.0 ± 0.3

70

M. Sharma · M.S. Eisen

Fig. 26 Crystal Structure of (η5 -C5 Me5 )2 Th(η4 -C4 H6 ) [264]. Reprinted with permission from [264];  (1986) American Chemical Society

(394), 15.0 ± 0.3 (299), and 10.5 ± 0.3 (208) kcal mol–1 for the complexes Cp∗ 2 U(η4 -C4 H6 ), Cp∗ 2 Th(η4 -C4 H6 ), and Cp∗ 2 Th(η4 -CH2 CMeCMeCH2 ), respectively [264].

Scheme 23 Rapid ring inversion in actinide diene complexes [264]

The uranium contingent of this butadiene molecule was characterized by NMR spectroscopy. At room temperature, the NMR data is consistent with a folded metallacyclopentene structure having magnetically nonequivalent Cp∗ ligands and α-methylene protons [264] as shown in the structure below:

Metallocene Organoactinide Complexes

71

The actinide–carbon bonds in these complexes appear to be reasonably polar; hence they undergo facile hydrogenolysis and protonolysis, but do not undergo the activation of C–H bonds on the exogenous hydrocarbon molecules. Hydrogenolysis with one atmosphere of dihydrogen yielded the dihydride complexes [264] (Eq. 57):

(57) The dimeric formulation of the dihydride complexes is well supported by both cryoscopic molecular weight determinations and a single crystal neutron diffraction structure of the thorium compound [285]. A rapid exchange between the bridge and terminal hydrides was observed over the NMR time scale in solution at – 85 ◦ C. In the case of uranium, the ring methyl protons appear to interchange rapidly with the hydrides, resulting in isotopic scrambling. The thorium complex is thermally stable whereas, in contrast, the uranium complex loses dihydrogen at room temperature in vacuo over a period of 3 h to generate a U(III) hydride. Thermochemical investigations have been carried out to find out the bond disruption enthalpies for a number of metallocene alkyl halide and dialkyl complexes [286, 287]. The Th–R bond enthalpies are uniformly larger than those for U–R. The bond dissociation enthalpy for Th–H in {(η5 -C5 Me5 )2 Th(µ-H)H}2 was observed to be 407.9 ± 2.9 kJ mol–1 , which is somewhat larger than for the typical Th–C values, 300–380 kJ mol–1 , but not larger enough to produce a strong driving force for the formation of hydrides. Therefore, unlike mid- to late-transition metal compounds, reactions such as β-hydride elimination will not be strongly favored and hence expected to affect C–C bond forming reactions, like olefin polymerization. Similar to the tris(cyclopentadienyl)actinides, bis(cyclopentadienyl)actinide complexes also undergo insertion reaction of various unsaturated substrates such as CO, CNR, CO2 , and CS2 into U–C, U–Si, U–N, and U–S bonds [221, 249, 265, 273, 284] and commonly have η2 -C(R) = E types of bonding and might exists with a “carbene-like” structure [288]:

72

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The syntheses, structures, and cyclometallation reactions of a series of bis(pentamethylcyclopentadieny1)thorium dialkyl complexes of the type Cp∗ 2 Th(CH2 EMe2 R)2 (where E = C, R = Me, Et; E = Si, R = Me, Ph) yielded the thoracyclobutane Cp∗ 2 Th(η2 -CH2 EMeRCH2 ) (Scheme 24) [289]. The complex Cp∗ 2 Th(η2 -CH2 SiMePhCH2 ) undergoes further thermolysis to form Cp∗ 2 Th(η2 -CH2 SiMe2 –o-C6 H4 ). An interesting results was observed when thermolysis was carried out with Cp∗ 2 Th(CH2 CMe3 )(CH2 SiMe3 ), which leads to the formation of Cp∗ 2 Th(η2 -CH2 SiMe2 CH2 ) by an intramolecular elimination reaction.

Scheme 24 Thermometallation of bis(cyclopentadienyl)thorium dialkyl complexes [289]

The mechanism for cyclometallation was proposed to involve a concerted heterolytic process with hydrogen atom abstraction and metallacycle formation occurring in a four-center transition state. Kinetic and labeling studies in the cyclometallation reactions indicate that intramolecular γ-C–H activation is the rate-limiting step [290]. The high reactivity of bis-arylactinide complexes can be observed by the benzene elimination that takes place to form actinide benzyne-type complexes, which further undergo reverse reaction of benzene to form odiphenylene (Scheme 25) [249].

Scheme 25 Elimination of benzene from diaryl complexes [249]

The benzyne intermediate for the uranium is unstable and, hence, the uranium complexes undergo a much faster ortho-activation process than the corresponding thorium complexes.

Metallocene Organoactinide Complexes

73

More recently it has been observed that Cp∗ 2 UI2 reacts with KCN and NR4 CN to give the familiar bent sandwich complex [Cp∗ 2 U(CN)3 ][NR4 ] and the pentacyanide complex [Cp∗ 2 U(CN)5 ][NR4 ]3 (where R = Et, n-Bu) (Eq. 58) [291]:

(58) [Cp∗

The complex 2 U(CN)3 ][N–Bu4 -n] was characterized crystallographically and found to posses a bent sandwich-like structure. The average A A, respectively, is 0.1 ˚ U–Cp and U–(CN) distances 2.727(15) and 2.520(16) ˚ smaller than its U(III) analog is in good agreement with the variation in the radii of U4+ to U3+ ions. Recently, another class of ligand, N-heterocyclic carbene, has jumped into the race for σ-donor ligands. Among these, imidazole carbene has received more emphasis because of its tunable stability with the help of substituents in the ring as well as on the nitrogen atom. Addition of one molar equivalent of the heterocyclic carbene, C3 Me4 N2 (tetramethylimidazolylidine), to Cp∗ 2 UI(py) in toluene led to the immediate substitution of the pyridine ligand to give the carbene complex Cp∗ 2 UI(C3 Me4 N2 ) [292]. The metal coordination geometry was found to adopt the pseudotetrahedral arrangement as (Fig. 27) found in the series of complexes Cp∗ 2 AnXY. Apart from these bis or tris-cyclopentadienyl complexes, there was an early report of mono-Cp complex (Cp)UCl3 (DME) (DME = 1,2-dimethoxy-

Fig. 27 Crystal structure of Cp∗ 2 UI(C3 Me4 N2 ) [292]. Reproduced with permission from the Royal Society of Chemistry

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ethane) [293]. The coordination chemistry of mono-Cp actinide complexes has not yet been elaborated much and there are few reports of such compounds as they are highly reactive. The complex (Cp)UCl3 (DME) was initially prepared by reaction of UCl4 with Tl(C5 H5 ) in DME. After that, a number of other base adducts of the uranium mono-ring compound have been prepared using both mono- [294, 295] and bi-dentate bases [296]. Similarly, the complex U(BH4 )4 reacts with Tl(C5 H5 ) to yield (Cp)U(BH4 )3 [297], which in presence of Lewis bases redistributed to CpU(BH4 )3 L2 (where L = THF, DME, HMPA). On the basis of solution NMR investigation, the structure of the complex (Cp)U(BH4 )3 (THF)2 was proposed to be mer-octahedral with cis-THF ligands and a pentahapto cyclopentadienyl ring. This structure was confirmed for the complex (Cp)UCl2 (THF)2 , but a rigorous NMR study [298] showed that there is an equilibrium between two isomers in solution for a variety of base adducts of (Cp)UCl3 . Analogous compounds of the formula (Cp)AnX3 L2 (X = halide, NCS– ) have been produced for thorium [294], neptunium [51, 299], and plutonium [300]. Working with the tri-substituted cyclopentadienyl ligand, 1,2,4-trimethylsilylcyclopentadienyl, Edelman and coworker [301] reported the first mono tris-substituted actinide complex [U{η5 -C5 H2 (SiMe3 )3 -1,2,4}Cl2 (THF) (µ-Cl)2 Li(THF)2 ] and found to have an approximately octahedral environment around the U center, with four equatorial Cl– ligands, trans-axial sub-

Fig. 28 Crystal structure of [U{η5 -C5 H2 (SiMe3 )3 -1,2,4}Cl2 (THF)(µ-Cl)2 Li(THF)2 ][301]. Reprinted with permission from Elsevier

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stituted Cp and THF ligands (Fig. 28). In a similar reaction, the ThCl4 reacts with Na{η5 -C5 H2 (SiMe3 )3 -1,2,4} and Na{η5 -C5 H2 (SiMe3 )3 -1,2,4}(pmdeta) to produce [{{Th(η5 -C5 H2 (SiMe3 )3 -1,2,4)Cl3 }2 NaCl(OEt2 )}2 ] and [{Th(η5 -C5 H2 (SiMe3 )3 -1,2,4)Cl3 }(pmdeta)], (pmdeta = MeN(CH2 CH2 NMe2 )2 ), respectively [19]. The molecular structure of [{{Th(η5 -C5 H2 (SiMe3 )3 -1, 2, 4)Cl3}2 NaCl(OEt2 )}2 ] is quite interesting as: (i) it is tetranuclear, (ii) it contains two types of bridging chlorides, eight µ2 - and two µ3 -chlorine atoms. Each thorium center occupies a distorted octahedral environment and is bonded η5 - to an axial substituted-Cp ligand with a triply bridging chloride in the trans-axial position as shown below:

Grignard reagents were also found to be suitable for the formation of mono-ring pentamethylcyclopentadienylthorium and pentamethylcyclopentadienyluranium complexes by the reaction with their tetrahalides [302, 303]. Spectroscopic data indicate a meridional disposition of the chloride ligands in a pseudo-octahedral geometry. In addition, the trihalide base adduct also undergoes a similar type of reaction (Eq. 59) [302, 304, 305]:

(59) By this metathesis process the mono(pentamethylcyclopentadienyl) complex Cp∗ ThBr3 (THF)3 was synthesized [303] and its reactivity extensively studied (Scheme 26). Complexes Cp∗ ThBr3 (THF), Cp∗ Th(OAr)(CH2 SiMe3 )2 and Cp∗ Th 2 [η -OC6 H3 –(t-Bu)CMe2 CH2 ](OAr)(O=PPh3 ) were characterized by single crystal X-ray diffraction studies. Cp∗ ThBr(OAr)2 exhibits somewhat a distorted three-legged piano-stool geometry with Th–Cp∗ centroid, Th–O,

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Scheme 26 Synthesis and reactivity of mono cylopentadienyl thorium complex [303]

A, respectively. and Th–Br distances of 2.57(1), 2.16(1) (av), and 2.821(2) ˚ Cp∗ Th(OAr)(CH2 SiMe3 )2 also displays a three-legged piano-stool geomA. etry with Th–C distances to the alkyl groups of 2.460(9) and 2.488(12) ˚ Th–Cp∗ centroid and Th–O distances are very similar to those found in A, respectively. Cp∗ Th[η2 -OC6 H3 – Cp∗ ThBr(OAr)2 , at 2.53(1) and 2.186(6) ˚ (t-Bu)CMe2 CH2 ](OAr)(O=PPh3 ) features a distorted trigonal bipyramidal geometry about the metal center, with the Cp∗ ligand and the oxygen atom of the cyclometallated aryloxide ligand occupying axial sites (Cp∗ centroid– Th–O of 168.3(3)◦ ). The Th–C distance to the tert-butyl methylene group is A, while Th–O distances to the aryloxide and triphenylphosphine 2.521(12) ˚ A, respectively. oxide ligands are 2.199(7) (average) and 2.445(7) ˚

4 Conclusion and Perspectives In this review we have shown the unique properties of the actinide complexes and some of the many novel features such as multielectron oxidation– reduction, catalytic activities towards various organic conversions etc. The catalytic chemistry of the organoactinide complexes is new, demanding, and

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sophisticated. The ability to tailor a catalytic precursor of actinide complexes with controlled electronic and steric features is challenging and opens a new field in this branch of chemistry. The authors believe that in the next few years we will see a higher implementation of these complexes and new ones in demanding chemical transformations. Acknowledgements This research was supported by the Israel Science Foundation Administered by the Israel Academy of Science and Humanities under Contract 83/01-1. M.S. thanks the Lady Davis Trust for a postdoctoral Fellowship.

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Struct Bond (2008) 127: 87–117 DOI 10.1007/430_2007_078  Springer-Verlag Berlin Heidelberg Published online: 24 January 2008

Activation of Small Molecules by U(III) Cyclooctatetraene and Pentalene Complexes Owen T. Summerscales · F. Geoffrey N. Cloke (✉) Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9Q, UK [email protected] 1

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Synthesis of Mixed Sandwich U(III) Cyclooctatetraene and Pentalene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronic Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Activation of Small Molecules . . . . . . . . . . . . . . . . . . Dinitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . Deltate Formation . . . . . . . . . . . . . . . . . . . . . . . . . Oxocarbons from CO . . . . . . . . . . . . . . . . . . . . . . . Squarate Formation . . . . . . . . . . . . . . . . . . . . . . . . Functionalisation and Extraction of U(IV)-Bound Oxocarbons Carbon Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . Phosphorous Species . . . . . . . . . . . . . . . . . . . . . . . Other Small Molecules . . . . . . . . . . . . . . . . . . . . . .

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Abstract The low-valent complexes of uranium (i.e. those containing U(III) centres) are characterised as reactive, highly reducing species that can effect novel, and potentially useful, transformations of small molecules. In this chapter we review one particular class of these compounds – those supported by cyclooctatetraene and pentalene ligands – whose reduction chemistry has recently demonstrated novel and unexpected results, including the cyclooligomerisation of CO. The syntheses and structures of these compounds are presented, and their reactivity towards a variety of small molecules is examined and reviewed. The reactivity towards carbon monoxide is discussed in reference to the historical development of obtaining oxocarbons from CO. Keywords Activation · Cyclooctatetraene · Pentalene · Reduction chemistry · Small molecule · Uranium Abbreviations COT† 1,4-Bis(tri-isopropylsilyl)cyclooctatetraene COT Cyclooctatetraene Cp∗ Pentamethylcyclopentadienyl CpMe4 H Tetramethylcyclopentadienyl

88 CpMe Cp DFT HMPA Ln Me2 bipy Pent SCE tacn THF tmp Tp∗ TXP

O.T. Summerscales · F.G.N. Cloke Monomethylcyclopentadienyl Cyclopentadienyl Density functional theory Hexamethylphosphoramide Lanthanide 4,4′ -Dimethyl-2,2′ -bipyridine Pentalene Standard calomel electrode Triazacyclononane Tetrahydrofuran Tetramethylphospholyl 3,5-Dimethyl tris(pyrazolyl)borate Tetra-(3,5-dimethylphenyl)porphyrinato

1 Introduction The controlled activation of small, relatively inert molecules has been a constant theme in organometallic chemistry since the 1950s [1]. It is concerned with new transformations of chemical feedstocks that are cheap and readily available, and challenges chemists to develop a new chemistry of simple molecules whose formulations have been known for centuries, where the old chemistries may appear apparently “exhausted”. Current research is particularly focused on developing “green” chemistry – such as the utilisation of the planet’s alkane resources more effectively via C – H activation [2], the development of new methods to store and/or usefully transform the greenhouse gas CO2 [3], efficient methods of removing polluting CFCs with C – F activation [4], conversion of N2 into nitrogen-containing organic products [5, 6], and the use of renewable sources of CH4 and CO as C1 building blocks for pharmaceuticals, materials or fuels [7]. Many of the reported successful activation processes are powered by electron-rich metal complexes; recently, uranium(III) compounds have displayed high, and in some cases, previously unknown types of reactivity in this context [8, 9]. These exciting discoveries made over the last 10 years or so by a number of workers represent a renewed effort at understanding and exploring the often unpredictable nature of U(III) reactivity [10–13]. This chemistry ultimately attempts to combine the control afforded by an organometallic reaction with the sheer reductive power of an alkali metal, in order to achieve useful transformations of small molecules. Furthermore, at a time when escalating energy needs are putting nuclear power in greater demand, it is of clear importance to find industrial uses for depleted uranium, a major side-product of the nuclear industry. Somewhat overlooked by “mainstream” organometallic chemistry (there are practical difficulties associated with handling the highly oxygen- and

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moisture-sensitive complexes of the low-valent f -elements) organoactinide, and closely related lanthanide, chemistry has been developed at a much slower rate than the corresponding chemistry of the d-block [14, 15]. Pioneering work by the Evans and Andersen groups has previously shown that ring-substituted cyclopentadienyl ligands can be extremely effective for harnessing the reduction chemistry of low-valent f -element centres. In particular, a wealth of remarkable chemistry demonstrating many new small molecule transformations has been shown for Sm(Cp∗ )2 (THF)x (x = 0, 2; Cp∗ = pentamethylcyclopentadienyl) and U(CpR )3 (THF)x (R = alkyl, trialkylsilyl; x = 1, 0). This work has been comprehensively reviewed elsewhere [9, 14]. In f -element reduction chemistry, it is usual for two equivalents of a reducing species “M(L)” to react with a substrate “S” to give a sandwiched product of the type [M(L)]2 n+ (µ-S)2n– , in which each metal donates one electron (or sometimes more, vide infra) to give a doubly reduced substrate, which is held between the two monoxidised fragments. The simple electronic reduction to give (S)2n– is appropriate for some cases but reductive homologation of substrates to units of (S)x 2n– and transformations to totally new substrates are also well documented. Most of these reductions derive from either a pair of one-electron Ln(II)/Ln(III) or U(III)/U(IV) processes; however, two-electron U(III)/U(V) processes are also known and demonstrate a further interest in uranium reduction chemistry. Uranium(III) centres possess accessible higher oxidation states, and thus may in theory participate in one-, two-, or threeelectron reductions whilst the divalent lanthanides are limited to one-electron processes. Having said this, the U(IV) state is a relatively stable configuration for “soft” ligands under anaerobic conditions, and therefore most of these reductions simply involve a one-electron U(III)/U(IV) process. Maintaining the approach of using “soft”, sterically hindered carbocyclic ligands to engender high reactivity, the chemistry of U(III) has been investigated using the substituted, eight-membered carbocycles cyclooctatetraene (COT) and pentalene (a bicyclic analogue of COT), in conjunction with ′ a cyclopentadienyl co-ligand, to give a variety of mixed-sandwich COTR /CpR ′ R R and Pent /Cp U(III) complexes. Their reactivity towards small molecules has produced some unique and exciting results, which are the subject of this chapter.

2 Synthesis of Mixed Sandwich U(III) Cyclooctatetraene and Pentalene Complexes In order to reflect the order in which the research was carried out, chem′ istry of the PentR /CpR uranium systems will be discussed first. Pentalene is an eight-membered bicyclic carbocycle; the planar dianion is a 10π Hückel

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aromatic, analogous to COT2– . As a ligand in organometallic chemistry, it may be commonly considered to act as two fused Cp rings; however, it is also extremely versatile with bonding modes varying from a bent, fully coordinated η8 -mode to simply η1 -bound [16]. The analogy with Cp is more structural than electronic – pentalene has fewer “aromatised” electrons than two Cp rings (10 vs. 12) – but crucially, can engage in δ-type bonding with the f -elements when fully coordinated, allowing more electron density to be donated overall. This was demonstrated originally for COT in the “classic” sandwich compound uranocene U(η-COT)2 . The bonding in uranocene is found to consist of strong δ-type donation into vacant d- and f -orbitals, which are of similar energy in the 5f metal uranium [17]. Synthesis of the uranocene analogue, bis(pentalene) U(IV), was achieved in 1997 using the dipotassium salt of the 1,4-bis(tri-isopropylsilyl) derivative C8 H4 † [K]2 {† = 1,4-bis(tri-isopropylsilyl)} and UCl4 to give U(η – C8 H4 † )2 [18]. Potassium salts are commonly used in preference to lithium salts for salt metathesis reactions with f -element halides to avoid the formation of aggregated “-ate” complexes; the bulky silyl groups impart solubility and crystallinity to the resulting organometallic compounds, aiding both manipulation and characterisation, and can sterically protect reactive metal centres. The same benefits can of course be achieved by the use of alkyl substituents, with the added benefit of enhanced electronic activation (electronwithdrawing silyl vs. electron-donating alkyl). However, preparing multiply alkylated pentalene or COT ligands is difficult; it has only been very recently achieved by O’Hare et al. with hexamethylpentalene [19]. Although routes are known to 1,4-dialkyl and 1,3,5,7-tetraalkyl COT species, the syntheses are multi-step, low-yielding reactions [20, 21]. In comparison, the 1,4-disilylated derivatives may be easily prepared in good yield by addition of the appropriate silyl electrophile to either the COT or pentalene dianion, followed by double deprotonation with KNH2 (Fig. 1) [22]. The origin of the asymmetry of substitution in the COT† dianion is the stable arrangement of C = C double bonds in the triene precursor. DFT analysis on the unsubstituted compound U(η – C8 H6 )2 showed that the primary interaction between the ligand and the metal is of δ-symmetry, in a manner similar to that found in uranocene [23]. Comparison of photoelectron spectra gave evidence that the electron “richness” at the tetravalent centre of the uranium compounds varies in the order U(C8 H4 † )2 >U(COT)2 >U(Cp)4 , implying a similar trend in the electrondonating properties of the ligands. The dianionic ligands COT and pentalene act as stronger electron donors for uranium than the anionic Cp ligands, due to the additional δ-interactions, and the U(IV) compounds U(COT)2 and U(C8 H4 † )2 are extremely stable under anaerobic conditions. Use of the ligands with the lower oxidation state, U(III), would be expected to make these compounds even more electronrich than the U(CpR )3 (THF)x systems (i.e. more reducing) and, at least partly

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Fig. 1 Synthetic routes to silylated eight-membered carbocycles

for this reason, has consequently allowed us to observe new types of small molecule activation. The synthesis of UI3 (THF)4 in 1989 has undoubtedly opened up the field of low-valent organouranium chemistry, as it provides a readily prepared, reasonably soluble starting material for the synthesis of U(III) complexes [24]. However, the THF solvent molecules are irreversibly bound to the metal centre, which has created difficulties in synthesising “base-free” uranium compounds, i.e. those without coordinated Lewis bases (such as THF) that can otherwise bind strongly to the uranium centre and block the active site that is required for small molecule activation. For example, only the base-free versions of the compounds Sm(Cp∗ )2 and U(CpMe4 H )3 have shown any reactivity with N2 and CO (respectively); the THF adducts are unreactive [25, 26]. Both these oxophilic compounds were obtained by desolvation of the correspond′ ing THF adducts, therefore initial attempts at the synthesis of a PentR /CpR mixed-sandwich U(III) complex involved synthesis of the THF adduct and subsequent desolvation. Addition of one equivalent of C8 H4 † [K]2 to UI3 (THF)4 gave the U(IV) compound U(C8 H4 † )2 exclusively, the product of a disproportionation. The high stability of the bis(pentalene) sandwich compound means that it is often found to be a thermodynamic sink for many of these reactions. The compound U(Cp∗ )I2 (THF)3 is known [27], and addition of one equivalent of C8 H4 † [K]2 to the latter in THF gave the desired U(III) compound U(C8 H4 † )(Cp∗ )(THF) (1.THF; see Appendix for list of structures) in good yield (Cloke FGN, unpublished results). However, this compound proved resistant to thermal desolvation in vacuo, and decomposed (at > 130 ◦ C) before losing any bound THF. Therefore, the synthesis of a base-free starting material was clearly desirable. Base-free UI3 may be obtained in almost quantitative yield by using a modification of a method described by Corbett for the synthesis of basefree lanthanide triiodides [28]: a mixture of oxide-free uranium turnings and

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two equivalents of HgI2 are heated in a thick-walled tube, sealed in vacuo, at 320 ◦ C for 3 days. Use of excess HgI2 is acceptable as the tetravalent compound UI4 is unstable with respect to disproportionation to I2 plus UI3 [29]. The base-free material UI3 is a purple-black solid, insoluble in all common solvents; it may be readily converted to the semi-soluble UI3 (THF)4 by addition of THF. The lack of solubility of UI3 is not problematic as it is typically solubilised by reaction with potassium salts in solvents such as diethyl ether and toluene.

Structure 1

The synthesis of base-free U(C8 H4 † )(Cp∗ ) (1) was achieved by the reaction of UI3 and KCp∗ in Et2 O, giving a green intermediate that was rigorously dried under vacuum, and subsequent addition of C8 H4 † [K]2 in toluene [30]. The U(III) compound, obtained in 40% yield after work-up, was found to be extremely air- and moisture-sensitive and could only be handled with the use of very high purity argon and freshly degassed, dry solvents. Structural determinations showed a slightly bent mixed-sandwich arrangement of fully coordinated ligands, requiring the pentalene ligand to adopt the bent η8 -mode. Although in the original paper describing this synthesis, the green intermediate material was assumed to be {UCp∗ I2 }n or an etherate thereof, more recent work has shown that the compound is in fact a mixed-valence U(III)/U(IV) trimer [U(Cp∗ )(µ-I)2 ]3 (µ3 -O) [31]. The oxo unit has been scavenged from the solvent, giving a cluster structurally very similar to the U(IV) sulfide [U(Cp∗ )I(µ-I)]3 (µ3 -I)(µ3 -S) (derived from U(Cp∗ )I2 (THF)3 and CS2 [32]). This illustrates the enhanced reducing power of the THF-free U(III) centres as U(Cp∗ )I2 (THF)3 will not activate Et2 O.

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It was envisaged that subtle variations in the steric and/or electronic prop′ erties of the PentR /CpR U(III) reducing agent might significantly affect the activation of small molecules, as observed in other systems. For example, Schrock has reported that slight variations in the substitution of terphenyl rings attached to a triamidoamine ligand significantly affect the outcome of dinitrogen activation in molybdenum systems [33, 34]. Thus, the synthesis of the less sterically hindered tetramethyl-Cp derivative was attempted, but resulted in the formation of the U(IV) dimer [U(C8 H4 † )(CpMe4 H )(µ-I)]2 (2) [35]. Presumably this tetravalent compound is generated from a disproportionation reaction of the type already described for the synthesis of the U(IV) tris-amide U(I)[N(t Bu)(1,3-C6 H3 Me2 )]3 from UI3 (THF)4 [36]. This result implies that there is a level of steric unsaturation above which these mixed-sandwich molecules become unstable as trivalent compounds. In order to further investigate these mixed-sandwich systems, the synthesis of the COT analogue, viz. U(COT† )(Cp∗ ), of the pentalene compound was explored. Unlike pentalene, COT is a commonly used ligand in organometallic chemistry, and has been much more thoroughly researched as such. The unsubstituted complex U(COT)(Cp∗ )(THF) (3.THF) was first reported by Sattelberger et al. in 1993 [37]; crystallographic characterisation remains elusive; however, adducts of the type U(COT)(Cp∗ )(L) (3.L) have been structurally characterised for L = Me2 bipy and HMPA, and show typical η8 - and η5 - coordination of the rings [37, 38]. A tetramethylphospholyl (PC4 Me4 ; tmp) derivative, U(COT)(tmp)(HMPA)2 [38], is also known and would be of interest in this context as tmp is closely related to CpMe4 H (C5 Me4 H), as borne out by the isostructural nature of the U(IV) compounds U(tmp)3 Cl and U(CpMe4 H )3 Cl [39, 40]. Attempts at repeating the synthetic route for base-free 1 from UI3 , using COT† [K]2 in the place of the pentalene salt, failed for reasons unknown. Therefore, the adduct U(COT† )(Cp∗ )(THF) (4.THF) was prepared in THF with the intention of removing the bound solvent molecule in vacuo. A onepot reaction of UI3 with KCp∗ in THF, and COT† [K]2 , furnished 4.THF as a THF adduct in moderate yield [41]. Further variants were synthesised with

Fig. 2 Synthesis of mixed-sandwich compounds 3.THF–6.THF from UI3

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Structure 2

Structure 3

the tetra- and monomethylated Cp rings, giving U(COT† )(CpMe4 H )(THF) (5.THF) and U(COT† )(CpMe )(THF) (6.THF), respectively (Fig. 2) [35, 42]. Use of the Cp analogue tris(3,5-dimethylpyrazolyl)borate (Tp∗ ) gave the base-free uranium complex U(COT† )(Tp∗ ) (7) despite the use of THF, presumably due to the extra steric crowding exerted by the Tp∗ ligand (Farnaby J, Hitchcock PB, Cloke FGN, unpublished results). The structure of 7 shows full η8 - and η3 -

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coordination of the ligands, with a non-linear centroid(COT† )–U–B angle, i.e. a slightly bent arrangement between the COT† and Tp∗ ligands. No uranocene impurities were detected in the isolated samples, although it was detected in the mother-liquor; its formation appears to limit the yield of these reactions. This novel uranocene U(COT† )2 , which may be obtained directly from UCl4 , is found to be too soluble to crystallise from pentane, unlike the trimethylsilyl analogue U(COT1,4-SiMe3 )2 , which has been found to crystallise readily from hydrocarbon solvents and contaminates samples of the less sterically hindered U(COT1,4-SiMe3 )(Cp∗ )(THF) (8.THF), limiting the utility of the bis-trimethylsilyl derivative in reduction chemistry [43]. X-ray analyses of 4.THF–6.THF revealed a typical bent, fully coordinated η8 - and η5 -arrangement of the rings in the COT† /CpR complexes, and also A), larger than any some very long U–O(THF) distances (average 2.710(4) ˚ others in the literature for a U(III) species. These values reflect a very weak bond, and it was consequently discovered that they could all be desolvated without decomposition at 100–110 ◦ C and 10–6 mbar to give U(COT† )(Cp∗ ) (4), U(COT† )(CpMe4 H ) (5) and U(COT† )(CpMe ) (6) [35]. The base-free compounds were all found to be extremely soluble (and very reactive) and could not be crystallised, therefore structural data are currently lacking; however, they have been characterised by 1 H NMR spectroscopy and mass spectrometry. Attempted desolvation of the less sterically hindered 3.THF resulted in thermal decomposition (at > 130 ◦ C). 2.1 Electronic Spectroscopy Electronic spectra for the f -elements often contain many weak, sharp peaks in the visible to near-infrared region; these originate from Laporteforbidden f –f transitions, and are observed in the late-visible/near-infrared (700–1200 nm) for solutions of these U(III) complexes [29]. The ground state of trivalent uranium is 5f 3 ; however, the 5f 2 6d state is sufficiently low-lying for 5f 3 → 5f 2 6d transitions to occur in the early-visible region, often in the range 15 000–30 000 cm–1 (ca. 350–650 nm). These transitions are parityallowed, often observed as broad, intense absorption peaks, and give U(III) compounds the dark, strong colorations that are distinctive of the oxidation state. These are noted as two main peaks for 4.THF, 5.THF and 6.THF at similar wavelengths; in 4.THF these occur at 497 and 584 nm (Summerscales OT, Hitchcock PB, Cloke FGN, unpublished results). These parameters are important as they allow a simple method of testing the oxidation state of the compounds. The higher U(IV) and U(V) oxidation states have more stabilised f -orbitals with respect to the d-orbitals, therefore the energy gap (i.e. 5f 2 → 5f 6d for tetravalent centres) is larger and hence the transitions occur in the UV part of the spectrum. Thus, U(IV) and U(V) are observed to be less darkly colored and more translucent in solution than the U(III) complexes.

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3 Activation of Small Molecules 3.1 Dinitrogen Dinitrogen activation at any metal centre is very challenging; the N≡N triple bond is strong (945 kJ mol–1 ) and apolar, restricting both bond-breaking and coordination mechanisms [5, 44]. These difficulties are compounded for the f -elements by the high nodality and poor radial extension of the f -orbitals, which are ill-suited for π back-bonding [15]. Nonetheless, a small number of f -element N2 complexes have been reported, most of which demonstrate the side-on activation mode. This is a noteworthy feature, given that this bonding mode has been recently demonstrated to be highly active towards further functionalisation (e.g. hydrogenation with H2 to give NH3 ) using other metals such as zirconium [44–46]. Intriguingly, uranium metal was used as a catalyst in the original Haber–Bosch reaction chambers [47]. Exposure of U(C8 H4 † )(Cp∗ ) to atmospheric pressures of dinitrogen generated a new species, the U(IV) dimer [U(C8 H4 † )(Cp∗ )]2 (µ-η2 :η2 -N2 ) (9), in which crystallographic studies revealed a side-on bound N2 unit bridging two opposing uranium fragments [30]. The binding of N2 is reversible and does not go to 100% completion. The dimer is in equilibrium with the U(III) monomer (Fig. 3), and even under 50 psi of dinitrogen, the reaction only goes to ca. 75% completion. The N2 is bound loosely, and is easily lost under vacuum in both solid and solution states. Both structural and theoretical data are consistent with a doubly reduced (N2 )2– moiety. The N – N distance in the crystal structure was found to be in agreement with a double A. DFT studies performed on the unsubstituted compound bond at 1.232(10) ˚ (in order to reduce computational time) were consistent with 5f 2 centres, i.e. a U(IV) configuration, with two electrons in a molecular orbital derived from the N2 πg orbitals and the uranium 5f and 6d orbitals [48]. This implies a sig-

Fig. 3 Reversible binding and reduction of N2 by 1

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Structure 4

nificant π back-donation from the U(IV) 5f and 6d orbitals to give a formal dianionic unit of (N2 )2– . The case is not so clear for the first uranium N2 compound, synthesised by Scott and co-workers in 1998, the triamidoamine complex [{NN3 }U]2 (µ-η2 :η2 -N2 ) (NN3 = N(CH2 CH2 NSit BuMe2 )3 ) [49]. The N – N A), A) is similar to that found in dinitrogen gas (1.0975 ˚ distance (1.109(7) ˚ and the UV/vis. spectrum is very close to those to that of the U(III) parent [{NN′3 }U]. Therefore it was concluded that no reduction of N2 has occurred; however, the uranium centres would have to act as an extremely strong Lewis acid to maintain this dimeric structure, which is formed in quantitative yield (as opposed to being part of an equilibrium state, vide supra). The compound loses N2 under vacuum. Computational studies, conversely, indicate that the most stable structure of the dimer contains a reduced N2 moiety with U(IV) 5f → πg back-donation, similar to the pentalene compound above [50]. The first lanthanide N2 compound, [Sm(Cp∗ )2 ]2 (µ-η2 :η2 -N2 ), shares features of both these uranium compounds. The N2 is bound weakly, in equilibrium with its monomer, as with 9; however, although crystallographic data show no lengthening of the N – N bond in the solid state, 1 H NMR evidence indicates two Sm(III) centres in solution [25]. It should be noted that the X-ray crystallographic data for this, and the triamidoamine uranium compound may be inaccurate, especially considering that the difference between N≡N and N = N is so small. Raman spectroscopy could be used to help clear up these discrepancies; however, spectra have not as yet been reported.

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These studies are of both academic and practical interest. A vital step for a catalyst for converting N2 into useful products is the electronic reduction of the bound molecule, and therefore it is important to gauge how actively the f elements are capable transferring electrons to N2 , if any such homogeneous catalysts are to be synthesised. Homometallic f -element systems generally do not demonstrate activation beyond (N = N)2– . However, heterometallic systems incorporating alkali metals have shown higher activation to give (N – N)4– and even complete cleavage to form nitrides [51–54]. End-on binding, fundamental to the Chatt cycle and the catalytic molybdenum(III) system reported by Schrock et al. [33], would be expected to be less preferable for uranium, due to the contracted f -orbitals. Nevertheless, one example has been recently shown of an end-on bound U(III) complex. U(Cp∗ )3 (η1 -N2 ) was prepared by Evans et al. using an 80 psi overpressure of N2 [55]. The N2 bond length is basically unchanged, therefore it is concluded that no formal reduction has occurred. However, data from Raman spectroscopy show the νN2 stretching frequency at 2207 cm–1 (versus the 2331 cm–1 reported for free N2 ), therefore indicating that electron density has been transferred to the ligand. The compound readily loses N2 . None of the base-free uranium COT† /CpR compounds 4, 5 or 6 gave any changes in the 1 H NMR or UV/vis. spectra when reacted with 1–2 bar N2 , compared with those obtained under argon. The tris-aryloxide-tacn (tacn = triazacyclononane) U(III) systems described by Meyer et al., although highly reactive to many types of small molecule, similarly do not show any reactivity with N2 , even with 80 psi overpressure [56]. The difference between the pentalene complex 1 and the COT complexes 4–6 may be explained by both the higher levels of steric congestion in the latter, and the increased flexibility of the η8 -pentalene ligand in the former, which folds at a decreased angle in order to create space for the dinitrogen moiety. Such folding has not been observed with η8 -COT bound to a single metal centre, and fully coordinated COT ligands have only been observed to bend in metal–metal bonded dimers, e.g. [M(L)]2 (µ:η5 :η5 -COT) (L = Cp, M = V, Cr; L = COT, M = Ti, Cr, Mo, W) [57–60]. 3.2 Carbon Monoxide Although a stronger σ-donor than isolectronic dinitrogen, carbon monoxide is also a better π-acceptor, a factor which similarly limits its capabilities as a ligand in f -element chemistry [15]. Its triple bond is likewise very difficult to cleave and kinetically inert (formally the strongest bond in nature at 1076 kJ mol–1 ). Its polarity and susceptibility to oxidation, however, allow a wealth of chemistry not easily observed with N2 , including the facile formation of transition metal carbonyl compounds, in which strong bonds result from σ-donation and π back-bonding [1]. Conversely, the factors that pro-

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mote transition metal-based reactivity generally limit lanthanide or actinidebased activation, as this polarity – reflected in the concentration of electron density found at the carbon atom – renders the molecule a soft σ-donor, and it thus binds poorly with the hard acceptor f -elements. Actinide carbonyl complexes were historically of interest for isotope separation under the Manhatten Project during the Second World War, because M(CO)x complexes of the transition metals are volatile [17]. These investigations failed to produce any stable carbonyl compounds, although U(CO)6 has since been characterised at liquid helium temperatures (it decomposes upon warming above 20 K) [61, 62]. Only one class of stable f -element carbonyl complex has been characterised in over 60 years of research, U(CpR )3 (η1 -CO) (R = SiMe3 , CMe3 , Me4 H, Me5 ), first prepared by Andersen et al. by exposing base-free U(CpSiMe3 )3 to CO [26, 63–65]. The reduced values of νCO in the IR spectrum give evidence for U – CO back-bonding, and therefore noticeable 5f participation. The CO is weakly bound and lost upon exposure to vacuum, although two of this series (R = Me5 and Me4 H) have proved stable enough to be crystallographically characterised, and show a typical terminal M – CO moiety [26, 65]. Reduction of CO is thermodynamically challenging and is known for just three examples from the f -block. Sm(η-Cp∗ )2 (THF)2 reacts under 90 psi overpressure CO to give a dianionic ketene carboxylate unit, (O2 C – C = C = O)2– , the product of a 2e– reduction [66]. The mechanism for the formation of this unit is unclear, and the chemistry is unique. Very recently it has been shown that [La(η-Cp∗ )2 ]2 (µ-N2 ) will reduce CO to give an identical ketene carboxylate unit, with loss of N2 [67]. The tris-aryloxide-tacn complex {(tBu ArO)3 tacn}U has been shown to react under room temperature and pressure with CO to produce [{(tBu ArO)3 tacn}U]2 (µ:η1 :η1 -CO), bridged by a mono-anionic (CO)– unit – an unusual dimerisation in that only one of the two uranium centres has been oxidised [68]. The bridging unit resulting from one-electron reduction of CO gave the first evidence that U(III) was capable of reducing carbon monoxide. Reaction of the pentalene compound 1 with 1 bar CO in pentane gave a translucent red solution, indicative of oxidation to U(IV). Mass spectral analysis of the product showed strong peaks at m/z = 1818 and 1846, corresponding to [1]2 (CO)3 + and [1]2 (CO)4 + , but in the absence of structural data the identity of the product(s) remains unclear (Cloke FGN, unpublished results). 3.2.1 Deltate Formation Although no reaction was observed between U(COT† )(Cp∗ ) and N2 , CO showed remarkable reactivity under mild conditions. Exposure of the darkly colored solutions of either base-free 4 or 4.THF to ambient pressures of CO ir-

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Fig. 4 Reductive cyclotrimerisation of CO by 4.THF to give the deltate dianion in 10

reversibly formed the dark red crystalline U(IV) dimer [U(COT† )(Cp∗ )]2 (µ: η1 :η2 -C3 O3 ) (10) in 40% isolated yield (Fig. 4) [41]. The structure showed the two uranium centres to be bridged by a planar, carbocyclic (C3 O3 ) unit. DFT studies and electronic spectroscopy gave strong evidence that the metal centres were tetravalent and therefore this unit, derived from a two-electron reductive cyclooligomerisation of CO, is formally a dianion. The deltate dianion is the first member of the aromatic oxocarbons, described in detail below. This reaction provided the first selective synthesis of the deltate dianion from CO, and the first crystallographic study of a deltate salt. Thorough investigation demonstrated that this reaction proceeds in both non-coordinating solvents (pentane, toluene) and coordinating (THF) solvents (with or without prior cooling) to give 10 [35]. Atmospheric pressures of CO were used, but sub-atmospheric, and even stoichiometric quantities of CO will react to form 10. It was shown that the adduct 4.THF reacts

Structure 5

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Structure 6 (top view)

in an identical manner to 4, albeit with facile loss of THF, and in similar yield. Although the use of a base-free compound would be thought to be a pre-requisite for carbonyl formation, this reduction and indeed all of the reductions reviewed in the rest of this article proceed as readily whether the U(III) centres are solvated or not. In fact, the reactions proceed when using THF as solvent. The structure of 10 contains a novel C – C agostic bond, resulting from σ-donation from one C – C bond in the deltate ring (on the η2 -bound portion of the oxocarbon) into an empty 5f orbital on the uranium. This has been shown in a DFT study, and observed experimentally by a lengthening of the corresponding C – C bond distance, and concomitant lengthening of the adjacent C – O distances [41]. This is the first example of this type of bonding for an f -element, but it is known in other areas of the periodic table, e.g. in NbClTp∗ (c-C3 H5 )(MeCCMe) [69], LiOC(Me)(c-C3 H5 )2 [70] and [Rh(Pi Pr3 )(C14 H16 )][BAr4 F ] [71]. Three-membered rings seem especially susceptible to this type of bonding, as the tight C – C – C bond angles in the triangular skeleton lead to non-optimal overlap of the hybridised orbitals, so destabilising the C – C σ-orbitals [72]. The relative instability of these orbitals is manifested in the corresponding difficulty in synthesis of three-membered carbocycles such as deltate. The agostic interaction in 10 may be in part responsible for stabilising the deltate dianion, enabling this facile synthesis from CO. Due to the quantity of unoccupied, highly-nodal f -orbitals, these compounds may be especially well-suited for effecting this reduction of CO under mild conditions. It is unlikely that the less radially diffuse 4f orbitals would be capable of supporting this type of agostic bond. Evans has commented that the ketene carboxylate obtained by Ln(II) reduction of CO (vide supra) may be a ring-opened derivative of the deltate [67]; however, it should be noted that no evidence for this pathway has yet been given.

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3.2.2 Oxocarbons from CO The oxocarbon series (CO)n 2– were first formally recognised during the 1960s by West and co-workers, following the discovery of squaric acid (the conjugate acid of the squarate dianion) in 1959 (Fig. 5) [73, 74]. They are all dianionic, comprising a carbocyclic skeleton with oxygen attached by bonds of intermediate bond order, and have been shown to be planar, and hence highly aromatic; the level of delocalisation decreases as the ring size increases. Despite their relatively recent recognition as members of a homologous aromatic series, salts of croconate and rhodizonate (n = 5 and 6) were first isolated in 1825 by Gmelin, in a solid state reaction of carbon with KOH at high temperatures [75]. The oxocarbons are reduced oligomers of carbon monoxide, and work by Liebig in 1834 first demonstrated that these carbocyclic compounds can be synthesised directly from CO; this was achieved by heating potassium metal in the presence of CO at 180 ◦ C [76]. Again, as Gmelin found, mixtures of croconate and rhodizonate were isolated. In 1837 Heller deduced that the rhodizonate was in fact a precursor to the smaller croconate [77]. This was definitively proved in 1887, with the discovery that oxidative ring-contraction of (C6 O6 )2– gives (C5 O5 )2– , a reaction that proceeds quantitatively using either O2 in alkaline aqueous solution, or MnO2 [78]. This is a very effective method of preparing croconate, and to date, no other synthesis is known [74].

Fig. 5 Known routes to the oxocarbons via reductive cyclooligomerisation of CO (uranium reactions discussed in this chapter not included)

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Low temperature studies of the reaction of the alkali metals with CO in liquid ammonia (at –50 ◦ C) were shown to give salts of the reactive ethyne diolate dianion (OC≡CO)2– , which are found to be explosive when dry [79– 81]. Upon heating, these salts trimerise to give the hexaanion (C6 O6 )6– , which oxidises rapidly in air to give the rhodizonate dianion (C6 O6 )2– (still the largest oxocarbon known) [82]. Although the mechanism for Liebig’s preparation of rhodizonate has not been proven (vide supra), these results strongly suggest that the formation proceeds via the reactive ethyne diolate dianion, formed from initial 2e– reduction of CO. High pressure electrochemical methods developed in the 1970s have yielded salts of squarate, in approx. 35% yield at 400 bar in polar solvents such as THF and DMF [83, 84]. Further work in the late 1990s showed the reaction may proceed in similar yields under 10 bar pressure, but no products were obtained below this pressure [85]. In separate work using similar high-pressure systems, a concerted mechanism has been proposed for these cyclisations involving simultaneous electron transfer and bond formation between four molecules of surface-bound CO [86]. Similar investigations of the reaction of alkali metals with CO under high pressures did not yield any squarate salts. Further electrochemical experiments involving CO, conducted in liquid ammonia at –50 ◦ C, gave the ethyne diolate [87], cf. the reactions with alkali metals in liquid ammonia. This reduction occurred at around –2.3 V vs. SCE, whereas the high pressure routes described above were reportedly in the range –2.2 to –2.6 V vs. SCE. These are very high reduction potentials, approaching those of sodium and potassium metals (–2.7 V and –2.9 V, respectively). Deltate is the most difficult oxocarbon to access, due to its highly strained three-membered ring, and to date there are only two reported “organic” preparations: a low-yield photolytic route from a di-silylated squarate, which undergoes ring contraction to give the di-silylated deltate [88], and the carbene insertion of CCl2 into di-tert-butoxyacetylene, which gives deltic acid after acidic work-up [89]. There is 13 C NMR evidence for the formation of trace amounts of deltate in a complex mixture of products from the reaction of CO with Na–K alloy in THF [90]; however, the U(III) synthesis described above is the first selective synthesis of this oxocarbon from CO [41]. Surface catalysis routes using alkaline earth oxides have yielded mixtures of various (CO)n 2– (n = 2–6 ) species from CO [91]. These routes are of mechanistic interest, but are of no synthetic value as only trace amounts of product are detected. Recent work has been reported that shows the formation of the rhodizonate mono-anion from the reaction of CO with molybdenum suboxide cluster anions Mox Oy – (y < 3x), which are generated using pulsed laser ablation/molecular beam methods [92]. The results suggest that a series of reactions occur involving the oxidation of CO until the oxygen content of the clusters is depleted, followed by metal carbonyl formation and, ultimately, free C6 O6 – formation.

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No mechanism for the formation of oxocarbons from various reductions of CO has been definitively proved. The most widely accepted notion is that 2e– reduction of two molecules of CO initially forms the ethyne diolate (OC≡CO)2– moiety, via combination of (CO· )– radicals, and consequent addition of neutral CO molecules forms progressively larger oxocarbons (CO)n 2– , the final product obtained depending on specific reaction conditions. However, it appears that the route Liebig pioneered using molten potassium at 180 ◦ C may proceed via cyclotrimerisation of ethyne diolate, and subsequent oxidation to give croconate and rhodizonate. Also there is the intriguing possibility of a concerted reduction pathway, whereby the oxocarbon is formed directly, e.g. a 2e– reduction of four molecules of CO to give squarate without intermediates. 3.2.3 Squarate Formation On the basis of decreasing ring strain from n = 3 to n = 5 in the oxocarbon series (CO)2 2– , it was proposed that higher homologues of deltate (n > 3) could be accessed with these U(III) systems by simply allowing a larger activation “pocket” for CO oligomerisation to occur following 2e– reduction. Indeed, tetramethylated 5.THF reacts with CO gave the squarate compond [U(COT† )(CpMe4 H )]2 (µ:η2 :η2 -C4 O4 ) (11) in isolated yields of up to 66% (Fig. 6) [42]. This is the first chemical synthesis of squarate from CO, and proceeds under mild conditions, unlike the pressurised electrochemical routes. Remarkably, the addition or removal of just one methyl group between the starting U(III) mixed-sandwich complexes 4↔5 allows selective access to either deltate or squarate, respectively, directly from CO.

Structure 7

The effect of such a slight change in ligand environment has been previously noted for Cp∗ vs. CpMe4 H , e.g. strikingly in zirconocene–dinitrogen

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Fig. 6 Formation of squarate from CO by 5.THF

systems, where the CpMe4 H ligand allows side-on binding and consequent hydrogenation of N2 to occur, whereas the use of Cp∗ prevents side-on binding and renders the (N2 )2– units inert to functionalisation [46, 93–95]. The squarate unit in 11, unlike the agostically bound deltate in 10, contains no geometrical distortions in the carbocyclic skeleton; DFT studies confirm the lack of the agostic bond in 11. This is presumably due to the lower ring strain in the four-membered ring. Gas phase SCF energies of the U(IV)-bound oxocarbon series [U(COT) (Cp)]2 (CO)n (n = 3–6) have been calculated, and are illustrated in Fig. 7. Energies of η2 :η2 -bound croconate and rhodizonate complexes were found to be

Fig. 7 Relative gas phase SCF energies of the U(IV) oxocarbons [U(η – COT)(η – Cp)]2 (CO)n ; n = 3–6

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lower in energy than any combination of η1 -bound modes [35] (Hazari N, Green JC, unpublished results). These calculations imply that the croconate should also be accessible in these systems by using an even less sterically hindered system. Unfortunately, the product(s) from the reaction of monomethylated 6.THF with CO could not be successfully crystallised (although readily soluble in all hydrocarbon solvents) and remain unidentified [35]. Reaction of the unsilylated compound 3.THF with CO gave a colour change to cherry red, distinctive of oxidation of the U(III) centres. However, the products were extremely insoluble and could not be purified. Thus the choice to use silyl groups for solubility factors has been shown to be a valid one. 3.2.4 Functionalisation and Extraction of U(IV)-Bound Oxocarbons The reductions described above may be considered the first syntheses of oxocarbons from CO under mild conditions (room temperature and pressure), and therefore give encouragement for the development of a useful (i.e. catalytic) process to generate these type of carbocycles from CO (a cheap and renewable feedstock) using a uranium catalyst. This type of reduction – a cyclooligomerisation of CO to give an oxocarbon product – is thermodynamically challenging and has only been previously demonstrated for the alkali metals. These metals suffer from a lack of solubility and selectivity, factors which have only been overcome up to now by using unattractive reaction conditions (high temperatures/pressures or liquid ammonia). The low-valent f -element complexes are characterised by a combination of solubilising ligands and high redox potentials, approaching those of the alkali metals. This is very clearly demonstrated by the U(III) complex 4, whose reactivity towards CO has only been otherwise observed using Na–K alloy as the reductant. Electrochemical evidence suggests that CO reduction requires highly reducing potentials in the range –2.2 to –3.0 V, so it is unsurprising that very few metal compounds have been shown to effect these reductions. The stabilising agostic interaction seen in 10 may be further responsible for enabling these reductions under mild conditions, and thus further limit this type of reactivity to metals that are suited to this interaction. The Fischer–Tropsch process utilises CO as a carbon source, with H2 as the reductant, for the production of hydrocarbons and oxygenates, especially in times of limited crude oil supply (CO may be derived from methane or coal). Fischer–Tropsch systems do not, however, give carbocyclic products, nor homologate CO under mild conditions (pressures typically used are >300 bar and temperatures >500 ◦ C, in conjunction with either homogeneous or heterogeneous catalysts [96]). Carbocycles often form the backbone of many pharmaceutical drugs, therefore a catalytic process that could synthesise them from a non-crude oil source (e.g. CO) under mild or even

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moderate conditions could be of considerable commercial and industrial importance [7]. Work by Wayland et al. has shown that the low-valent rhodium compounds such as [(TXP)Rh]2 (TXP = tetra-(3,5-dimethylphenyl)porphyrinato), can reduce CO to form ethane dionyl compounds of the type [(TXP)Rh-C(O)C(O)-Rh(TXP)] [97–100]. The late transition metals prefer to bind to carbon, and therefore this is formed in preference to an ethyne diolate product. Although carbocyclic products are not observed, the reactions demonstrate that reductive homologation of CO is feasible with transition metals under mild conditions. Related are the reactions of hydrosilanes with CO using rhodium and cobalt carbonyl catalysts, which give straight-chain products containing up to two or three coupled CO units (however, at elevated pressures and temperatures) [101, 102]. Clearly an important initial step towards designing and/or demonstrating the feasibility of a future catalytic process is to cleave the newly generated substrate from the metal centres. The labelled squarate compound 11-13 CO (generated from 13 CO) reacted cleanly with two equivalents of SiMe3 Cl to give the new compounds U(COT† )(CpMe4 H )Cl (12) and 13 C 4 O4 (SiMe3 )2 in quantitative yield (Fig. 8) (Summerscales OT and Cloke FGN, unpublished results). The free squarate in this case contained a fully labelled 13 C ring, and demonstrates a straightforward method of obtaining these rare organic compounds. Squaric acid itself may be obtained from C4 O4 (SiMe3 )2 by use of n-butanol [88]. Simple derivatives of squaric acid and the squarate dianion have many uses and are currently utilised in medicinal and biological chemistry, bioconjugate chemistry, materials science (using squarate to form conjugated polymers with low HOMO-LUMO gaps), dyes, photochemistry and organic synthesis (for ring expansions and in total synthesis) [103–112]. The deltate 10-13 CO also reacted with SiMe3 Cl to give U(COT† )(Cp∗ )Cl (13) [35]. However, the expected bis(trimethylsilyl) deltate derivative was not

Fig. 8 Synthesis of bis(trimethylsilyl)squarate from CO and SiMe3 Cl, with the uranium complex providing electrons for reductive oligomerisation and steric control; conversion to squaric acid previously reported

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detected and it is possible that it underwent an electrophilic ring-opening and was still bound to an, as yet, unidentified uranium species. Routes for “completing the cycle” back to the U(III) compounds, and other catalytic routes, are currently under investigation in our laboratory. 3.3 Carbon Dioxide Carbon dioxide is more easily reduced than CO, and reductive activation may be chemical [113, 114], enzymatic [115], electrochemical or photochemical [116, 117], proceeding by one-, two-, four-, six- or even eightelectron steps. Two-electron processes are the most common and are known to produce salts of formate, oxo, carbonate, and oxalate dianions, and carbon monoxide [118]. Reaction of 4.THF with 13 CO 2 gave 13 CO and an unidentified U(IV) or U(V) product. No 13 C peaks attributable to a uranium species were observed in the 13 C spectrum, and therefore it is most likely that the product is oxo (O)2– plus CO – an already reported reactivity pattern for the U(III) complexes U(CpSiMe3 )3 (THF) [119] and {(tBu ArO)3 tacn}U [68]. However, exposure of 5.THF to excess CO2 (1 bar) gave the structurally characterised U(IV) carbonate product [U(COT† )(CpMe4 H )]2 (µ-η1 :η2 -CO3 ) (14) and free CO (Summerscales OT, Hitchcock PB, Cloke FGN, unpublished results). This is a new CO2 reduction product for an actinide compound, but the formation of carbonate by this route is known for the transition metals and lanthanides [120, 121]. The transformation is a reductive disproportionation, involving a 2e– reduction of two molecules of CO2 to CO3 2– with concomitant formation of CO (Eq. 1): 2CO2 + 2 e– → CO3 2– + CO .

Structure 8

(1)

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109

Given that 5.THF has been shown to cyclotetramerise CO (vide supra) it is clear that it must react much faster with the excess of CO2 than the CO produced by the reductive disproportionation. However, if the correct stoichiometry of reactants is employed (5:4 5.THF:CO2 , as shown in Scheme 1), the evolved CO can also be reduced, so that the overall result is the formation of 14 and the squarate product 11 (Fig. 9).

Scheme 1 Worked justification for the 5:4 ratio of U(III):CO2 used to obtain highest yields of squarate

Fig. 9 Reductions of CO2 using U(III) cyclooctatetraene complex 5.THF

This is the first synthesis of an oxocarbon from a CO2 carbon source and its synthesis may be considered as the product of successive 2e– reductions of CO2 . The first reduction gives carbonate plus CO, the second then reduces this liberated CO to the squarate dianion (Eq. 2). However, it must be noted that

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Structure 9

the maximum yield of the squarate by this method is clearly never going to be higher than 20%: 8e–

2e–

8CO2 → 4CO3 2– + 4CO → 4CO3 2– + C4 O4 2– .

(2)

3.4 Phosphorous Species The reaction of 1 with phosphaalkyne P≡Ct Bu gave [U(C8 H4 † )(Cp∗ )]2 (µη1 :η2 -PCt Bu) (15), in which a bent PCt Bu moiety bridges the two uranium centres via the phosphorous atom (Summerscales OT, Hitchcock PB, Cloke FGN, unpublished results). The P – C – C angle is consistent with an sp2 hybridised centre, the product of a two-electron reduction of the phosphaalkyne. This is the first isolation of a phosphaalkyne dianion, although the electron-acceptor properties of PCt Bu previously studied by electron transmission spectroscopy have indicated its accessibility [122]. One-electron reductions of phosphaalkynes are known, and give homologated and cyclised compounds [123–126]. The analogous reactions using the COT compounds 4 or 5, however, did not lead to isolable products. White phosphrous (P4 ) is reduced by 4.THF and 5.THF to give [U(COT† ) (Cp∗ )]2 (µ-η2 :η2 -P4 ) (16) and [U(COT† )(CpMe4 H )]2 (µ-η2 :η2 -P4 ) (17), respectively (Frey ASP, Hitchcock PB, Cloke FGN, unpublished results). Two P – P bonds have been cleaved, resulting in a planar dianionic moiety that bridges the two centres. This cyclo-P4 form has been observed previously in the products of photochemical and thermal reactions of white phosphorous with

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111

Structure 10

transition metal carbonyl compounds [127–129]; the U(III) reactions give this product under ambient conditions. 3.5 Other Small Molecules Initial estimates of the reductive power of U(COT)(Cp∗ )(THF) were underwhelming; the redox potential was reported at E◦ {U(IV)/U(III)} = –0.69 V vs. SCE [37]. The reaction with dimethylbipyridine seemed to confirm this view, as a U(III) adduct 3.Me2 bipy was formed without reduction of the substrate, in spite of the relatively low-lying π ∗ acceptor orbitals of bipy [37]. Later work, however, showed that 3.THF could reduce neutral COT to give the U(IV) complex [U(COT)(Cp∗ )](µ-η3 :η3 -COT) (18) [130], in spite of the lower electron affinity of COT vs. bipy [131]. Clearly it is misleading to use these reduction potentials as a strict guide for measuring the reactivity of these compounds. It has been shown that the silylated derivatives 4 and 5 display reactivities with CO only previously observed with alkali metals (E◦ {Na(I)/Na(0)} = –2.7 V vs. SCE) [41, 42]. There is also mention in the literature of the reduction of CS2 to give [U(COT)(Cp∗ )]2 (µ:η1 :η2 CS2 ) (19), however, this has not been substantiated by any characterising data [32].

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None of the complexes 1, 4, 5 or 6 react with H2 under mild conditions [35], consistent with the observation that hydride ligands can be good reducing agents for U(IV) compounds [132–135]. Indeed, reaction of U(COT1,4-SiMe3 )(Cp∗ )(Me) (20) with H2 gave U(COT1,4-SiMe3 )(Cp∗ ) (8) plus methane [43]; we presume a transient U(IV) hydride complex [U(COT1,4-SiMe3 )(Cp∗ )(H)]x disproportionates to hydrogen and the observed U(III) compound. Reaction of 5.THF with MeCN gave no reduction products. A simple U(III) nitrile adduct U(COT† )(CpMe4 H )(η1 -NCMe) (5.MeCN) was formed instead, with loss of THF. Similar adduct formation is known for the triscyclopentadienyl systems; however, upon heating the latter, the ligand is reduced to give U(IV) cyanide and methyl products [136]. Thermolysis of 5.MeCN has yet to be investigated. 4.THF reacted with one equivalent of i PrNCO to give free isocyanide i PrNC and an unidentified oxidised uranium product – presumably an oxo species [35]. Reduction of isocyanates is known for one other U(III) system: U(η-CpMe )3 (THF) reacts with PhNCO to give the reduced bridged species [U(η-CpMe )3 ]2 (µ:η1 :η2 -PhNCO) [137]. The 2e– reduction of RNCO to RCN plus (O)2– appears to be unprecedented, although it is, perhaps, an unsurprising reaction for a low-valent f -element complex, given the noted oxophilicity of the latter.

4 Summary This article has reviewed the synthesis and reactivity towards small molecules of a range of U(III) cyclooctatetraene and pentalene complexes. It is evident that in many cases the uranium centre is capable of π back-bonding through the 5f orbitals; additionally, a C – C agostic interaction between a bound substrate and a U(IV) centre has been observed. Clearly uranium is capable of bonding with a degree of “covalency”, and this is perhaps why the reduction chemistry of U(III) is so rich and diverse, and not simply an iteration of low-valent lanthanide chemistry. A U(III) pentalene complex has been shown to be capable of binding and reducing N2 , in a similar manner to the divalent lanthanide complexes reported by Evans. However, the reactivity of the COT complexes towards CO is completely novel for organometallic compounds of any type, and shows considerable promise for the future design of a system capable of producing oxocarbon products from CO or CO2 catalytically.

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Appendix Compound

Number

Refs.

U(C8 H4 † )(Cp∗ ) U(C8 H4 † )(Cp∗ )(THF)

(1) (1.THF)

[U(C8 H4 † )(CpMe4 H )(µ – I)]2 U(COT)(Cp∗ )(THF) U(COT)(Cp∗ )(Me2 bipy) U(COT)(Cp∗ )(HMPA) U(COT† )(Cp∗ ) U(COT† )(Cp∗ )(THF) U(COT† )(CpMe4 H ) U(COT† )(CpMe4 H )(THF) U(COT† )(CpMe4 H )(η1 – NCMe)

(2) (3.THF) (3.Me2 bipy) (3.HMPA) (4) (4.THF) (5) (5.THF) (5.MeCN)

U(COT† )(CpMe ) U(COT† )(CpMe )(THF) U(COT† )(Tp∗ )

(6) (6.THF) (7)

U(COT1,4-SiMe3 )(Cp∗ ) U(COT1,4-SiMe3 )(Cp∗ )(THF) [U(C8 H4 † )(Cp∗ )]2 (µ-η2 :η2 -N2 ) [U(COT† )(Cp∗ )]2 (µ:η1 :η2 -C3 O3 ) [U(COT† )(CpMe4 H )]2 (µ:η2 :η2 -C4 O4 ) U(COT† )(CpMe4 H )Cl

(8) (8.THF) (9) (10) (11) (12)

U(COT† )(Cp∗ )Cl

(13)

[U(COT† )(CpMe4 H )]2 (µ-η1 :η2 -CO3 )

(14)

[U(C8 H4 † )(Cp∗ )]2 (µ-η1 :η2 -PCt Bu)

(15)

[U(COT† )(Cp∗ )]2 (µ-η2 :η2 -P4 )

(16)

[U(COT† )(CpMe4 H )]2 (µ-η2 :η2 -P4 )

(17)

[U(COT)(Cp∗ )](µ-η3 :η3 -COT) [U(COT)(Cp∗ )]2 (µ:η1 :η2 -CS2 ) U(COT1,4-SiMe3 )(Cp∗ )(Me)

(18) (19) (20)

[30] (Cloke FGN unpublished results) [35] [37] [37] [38] [35] [41] [35] [42] (Farnaby J, Hitchcock PB, Cloke FGN, unpublished results) [35] [35] (Farnaby J, Hitchcock PB, Cloke FGN, unpublished results) [43] [43] [30] [41] [42] (Summerscales OT and Cloke FGN, unpublished results) (Summerscales OT and Cloke FGN, unpublished results) (Summerscales OT, Hitchcock PB and Cloke FGN, unpublished results (Summerscales OT, Hitchcock PB, Cloke FGN, unpublished results) (Frey ASP, Hitchcock PB, Cloke FGN, unpublished results) (Frey ASP, Hitchcock PB, Cloke FGN, unpublished results) [130] [32] [43]

† = 1,4-bis(tri-isopropylsilyl)

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Struct Bond (2008) 127: 119–176 DOI 10.1007/430_2007_081  Springer-Verlag Berlin Heidelberg Published online: 26 February 2008

Highlights in Uranium Coordination Chemistry Suzanne C. Bart · Karsten Meyer (✉) Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University of Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany [email protected] 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

120

2

Entry into Uranium Coordination Chemistry: The First Convenient Uranium Starting Materials . . . . . . . . . . . . . .

120

Synthesis of Highly Reactive Uranium Precursors with Monomeric, Chelating, and Macrocyclic Ligands . . . . . . . . . . . .

122

4 4.1 4.2 4.3

High-Valent Uranium Complexes with Multiply Bonded Ligands . Complexes Containing the [O = U = O]2+/+ Subunit . . . . . . . . High-Valent Uranium Complexes with Nitrogen Donor Ligands . Unprecedented Uranium(IV) Coordination Complexes . . . . . .

. . . .

130 130 140 143

5

Uranium Coordination Complexes with Chalcogen-Containing Ligands . .

149

6

Multimetallic Systems of Uranium . . . . . . . . . . . . . . . . . . . . . . .

158

7

Recent Highlights and Perspectives . . . . . . . . . . . . . . . . . . . . . .

163

8

Closing Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

172

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3

. . . .

. . . .

. . . .

. . . .

Abstract The coordination chemistry of uranium has become an increasingly popular topic in the last 15 years. Much of the reason for this interest has come from the development of easy to synthesize, stable starting materials. These materials allowed an entry point for the exploration of uranium with any ligand imaginable. This chapter covers the most significant developments in the coordination chemistry of non-cyclopentadienyl uranium complexes and their reactivity with small molecules. Keywords Coordination chemistry · Molecular and electronic structure · Small molecule activation · Uranium Abbreviations Ac Acetyl bipy 2,2′ -Bipyridine Bu Butyl COT Cyclooctatetraene

120 Cp∗ CpMe4 dme DMF DMSO fc OTf phen py tacn terpy THF tppo ttcn

S.C. Bart · K. Meyer 1,2,3,4,5-Pentamethylcyclopentadiene 1,2,3,4-Tetramethylcyclopentadiene 1,2-Dimethoxyethane N,N ′ -Dimethylformamide Dimethyl sulfoxide Ferrocene Triflate 1,10-Phenanthroline Pyridine 1,4,7-Triazacyclononane Terpyridine Tetrahydrofuran Triphenylphosphine oxide 1,4,7-Trithiaazacyclononane

1 Introduction The coordination chemistry of uranium has undergone substantial growth in the last 10–15 years [1–8]. One impetus for this forward movement was the lack of knowledge about the bonding in f elements. Exploring uranium chemistry began to give new insights into the coordination behavior and bonding interactions of 5f elements, and allowed the exploration of ionic and covalent metal–ligand interactions. Studying topics such as these offers the potential for new complexes and catalytic applications. This chapter encompasses recent significant work on the structure and bonding of uranium coordination chemistry over the last 10–12 years. The complexes discussed have all types of molecular architectures with a wide array of donor ligands. Uranium complexes supported by cyclopentadienyl ligands have been reviewed elsewhere [1]. Exceptional examples have been included. A review has also appeared on uranium complexes with multidentate N-donor ligands [2]. Some of these complexes will be repeated as applications to other types of chemistry.

2 Entry into Uranium Coordination Chemistry: The First Convenient Uranium Starting Materials The generation of mid-valent uranium starting complexes has played a key role in the recent growth of uranium coordination chemistry. The synthesis of [UI3 (THF)4 ], originally reported by Clark and Sattelberger in 1994 [9– 11], was a spark that ignited interest in the chemistry of low-valent U(III) because it is operationally simple to make on a large scale with high yields. Cleaned and amalgamated uranium turnings are oxidized with elemental iod-

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ine in the presence of THF to produce a dark blue solid easily isolated by filtration. Recently, a procedure for solvent-free UI3 was reported, allowing this starting material to be used in solvent-sensitive reactions as well [12]. The more difficult synthesis of UCl3 involves using elemental uranium metal and hydrogen chloride gas at elevated temperatures of 250–300 ◦ C [13]. The trivalent uranium triflate analogue, [U(OTf)3 ], is synthesized by treating UH3 with an excess of triflic acid, HOTf, at 20 ◦ C to produce a fine green powder accompanied by liberation of hydrogen gas [14]. The next generation complex, [U(N(SiMe3 )2 )3 ], was initially reported by Andersen and synthesized by addition of three equivalents of the sodium amide to [UI3 (THF)4 ] (Fig. 1) [15, 16]. Ligand exchange has been studied with [UI3 (THF)4 ] by dissolution of this complex in other donating solvents. For instance, green crystals of the nine-

Fig. 1 Molecular structure of seminal uranium(III) complexes, [UI3 (THF)4 ] (right) and [U(N(SiMe3 )2 )3 ] (left). Hydrogen atoms omitted for clarity

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coordinate uranium(III) acetonitrile salt, [UI3 (MeCN)9 ] were obtained by addition of acetonitrile to [UI3 (THF)4 ] [17]. Significantly, this work demonstrates the ability to easily displace iodine from the coordination sphere through addition of N-donor ligands. Addition of two equivalents of 2,2′ bipyridine to [UI3 (py)4 ] furnished [UI3 (bipy)2 (py)] [18]. The coordination polymer [U(OTf)3 (MeCN)3 ]n was formed via salt metathesis of [UI3 (THF)4 ] with potassium triflate in acetonitrile [19]. Uranium(IV) halide complexes have also been synthesized recently. Elemental uranium is used for the synthesis of UBr4 and is treated with elemental bromine, while UCl4 is conveniently made from UO3 and hexachloropropene [20]. The synthesis of the solvent-free uranium(IV) analogue, UI4 , is accomplished by heating a mixture of uranium metal and iodine to 530 ◦ C [21]. Once synthesized, UI4 can also be isolated as various solvates, including [UI4 (MeCN)4 ] and [UI4 (py)3 ] [22]. The reaction of oxide-free uranium metal turnings with 1.3 equivalents of elemental iodine in benzonitrile provides [UI4 (NCPh)], a versatile U(IV) synthon which is soluble in organic solvents and has been fully characterized [23]. A volatile uranium derivative, [U(BH4 )4 ], is made by addition of Al(BH4 )3 to UF4 [24]. The uranium(IV) triflate compound, [U(OTf)4 ], was synthesized by heating UH3 and an excess of triflic acid to 180 ◦ C or by treating UCl4 with triflic acid at 120 ◦ C [14]. This compound has been used to make many different types of derivatives [25]. Uranium oxo derivatives of varying oxidation states have been widely used as starting materials. For instance, the synthesis of uranium(IV) oxide involves hydrogenation of U3 O8 to produce UO2 and H2 O. Uranyl starting complexes of the form [UO2 ][X2 ] are very important as an entry into U(VI) chemistry. These starting complexes include [UO2 Cl2 ], [UO2 (OTf)2 ], and [UO2 (NO3 )2 ]. Exposure of UCl4 to oxygen at 350 ◦ C produces uranyl dichloride, while uranium(VI) triflate can be made by addition of oxygen to U3 O8 . The latter is also formed by treating UO3 with triflic acid TfOH at 110 ◦ C or with boiling triflic anhydride, TfOTf, affording [UO2 (OTf)2 ] in high yields [26]. Synthesis is also possible by addition of UO3 to TfOH in water, or by dehydration of [UO2 (OTf)2 (H2 O)n ] in boiling TfOTf. Uranyl nitrate, [UO2 (NO3 )2 ], is commercially available and is synthesized by the addition of N2 O5 to UO3 .

3 Synthesis of Highly Reactive Uranium Precursors with Monomeric, Chelating, and Macrocyclic Ligands From these initial uranium starting materials, low- and mid-valent coordination complexes have been made with many molecular architectures, and vary depending on the type of ligands used. Because it is well documented that subtle changes in ligand sterics produce drastic changes in reactivity, choice

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of the proper ligand initially is very important [27–29]. The uranium centers in the complexes discussed here are either coordinatively unsaturated or have a labile neutral ligand, allowing these molecules to participate in further chemistry, including small molecule activation. Uranium(III) systems have been developed using a tris(anilide) framework [30]. The uranium(III) species [(N[t Bu]Ar)3 U(THF)] (Ar=C6 H3 Me2 -3,5) was isolated as a black solid (Fig. 2). 1 H NMR data at 25 ◦ C indicate a single ligand environment. Crystallographic characterization showed the U(III) comA. The ipso carbons of the adjacent plex has average U–N bond lengths of 2.320 ˚ phenyl rings are also closely associated with the uranium center, with U–C bond A. This is consistent with U(III)–arene π complexation [31]. lengths of ∼2.9 ˚ The molecular structure also shows an interesting feature: the THF ligand is located in the arene “bowl” above the uranium center, rather than in the “pocket” formed by the tert-butyl groups underneath the uranium [32]. A uranium derivative of the constrained geometry ligand has been synthesized, [(CGC)U(NMe)2 ] (CGC=Me2 Si(η5 –Me4 C5 )(t BuN)), containing a dimethylamide to complete the coordination sphere [33]. Crystallographic analysis of brown microcrystals revealed an η5 coordination mode of the cy˚ (Fig. 3). clopentadienyl ring and a U–N(L)(t Bu) bond length of 2.207(4) A ˚ The U–N(dimethylamide) distance is similar at 2.21(1) A. This compound has been demonstrated to perform intermolecular hydroamination/cyclization of amine substrates with unsaturated C–C tethers.

Fig. 2 Molecular structure of [(N[t Bu]Ar)3 U(THF)]. Hydrogen atoms omitted for clarity

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Fig. 3 Molecular structure of [Me2 Si(η5 –Me4 C5 )(t BuN)U(NMe2 )2 ]. Hydrogen atoms omitted for clarity

Similar to the famous uranocene complex, [(C8 H8 )2 U] [34, 35], a bispentalene uranium complex, [(η8 –C8 H4 (1, 4–Sii Pr3 )2 )2 U], was prepared via the reaction of UCl4 with two equivalents of [K2 (C8 H4 {1, 4–Sii Pr3 }2 )] [36]. The binding of the pentalene ligand in an η8 mode is examined by density functional calculations and photoelectron spectroscopy. The uranium complex is modeled as D2d symmetric and has a ground triplet state. The HOMO of the pentalene dianion is an orbital of δ symmetry with respect to an η8 coordinated metal. In this bispentalene complex, actinides provide both 5f and 6d orbitals that can overlap with the symmetry adapted linear combinations of these pentalene HOMOs and form covalent bonds. The HOMO-1 of the pentalene dianion is also able to form bonds with metal d and f orbitals, but with a smaller contribution from the metal. Overall, the 6d orbitals make a larger contribution to bonding than the 5f . The conformation of the ligand arrangement is dictated mostly by steric effects [36]. The twelve-coordinate, icosahedral U(III) complex, [(HB(3-(2-pyridyl)pz)3 )2 U], was reported in 1995 (Fig. 4). This complex features the typical tris(pyrazol-1-yl)hydroborate ligand substituted with a 2-pyridine group in the 3-position. Synthesis was achieved by salt metathesis, by addition of the potassium salt of the ligand to [UI3 (THF)4 ] [37]. This complex is the first example of an actinide species with N12 coordination, as both the pyrazolyl and pyridyl substituents are coordinated to the uranium center through their A, and are nitrogen atoms. The uranium–pyridyl bond lengths average 2.95 ˚ longer than the corresponding metal–pyrazolyl average distance of 2.66 ˚ A. The two ligands fit comfortably around the large metal center, as indicated by

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Fig. 4 Molecular structure of [(HB(3-(2-pyridyl)-pz)3 )2 U]. Hydrogen atoms omitted for clarity

the unstrained N–B–N bond angle of ∼109–110◦ . The bite angles of the N,N chelating fragments are between 57 and 59◦ [37]. Another variation on the typical tris(pyrazol-1-yl)hydroborate ligand has been synthesized in which two pyrazolyl units are substituted by two sulfurbased 2-mercapto-1-methylimidazolyl rings [38]. The cationic complexes [(κ 3 -H(R)B(timMe )2 )2 U(THF)3 ][BPh4 ] (R=H, Ph; timMe = (2-mercapto-1methylimidazolyl)borate) that were isolated and fully characterized are unprecedented cationic U(III) complexes anchored by poly(thioimidazolyl)borate ligands. Each ligand is bound in a tridentate fashion through two thione sulfurs and an agostic hydrogen interaction. The coordination mode is similar to that of the analogous nitrogen-based ligand [H(R)B(pz∗ )2 ]– (R=H, Ph; pz∗ = pz, 3,5-Me2 -pz, 3-t Bu-5-Me-pz) [39–41]. The phenyl derivative was crystallographically characterized (Fig. 5) and showed U–B distances of ˚. Crystallography confirmed agostic U–H bonds of 3.547(13) and 3.616(3) A length 2.61 and 2.71 ˚ A were calculated by positioning the H(B) atoms at their ˚. The C–S idealized location. The average U–S bond distance is 2.928(11) A ˚) are ˚ bond distances ranging from 1.668(12) to 1.726(12) A (avg. = 1.71(3) A intermediate of single and double bonds, indicating partial reduction of the π character of the C–S bond. The labile THF allows accessibility to the metal

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center, thus opening the way to a new class of uranium compounds via ligand substitution reactions. The 1 H NMR spectra of this class of molecules display a single set of three resonances for the four thioimidazolyl groups in the ratio 12(Me) : 4(Ha ) : 4(Hb ). The resonances attributed to the tetraphenylborate counterion appear as three signals near the diamagnetic region, integrating in a ratio of 8(Hc ) : 8(Hd ) : 4(He ). For the phenyl derivative, the aromatic rings appear also as two triplets and one doublet. The 1 H NMR spectrum is consistent with a fluxional molecule on the NMR timescale at room temperature.

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Uranium chemistry has also been explored with functionalized triazacyclononane frameworks. Several generations of uranium compounds have been developed using a triazacyclononane substituted with aryloxide groups containing bulky alkyl substituents [42]. The first generation system has ligands substituted with tert-butyl groups in the ortho position of the aryloxide ring, to afford the monomeric uranium complex [((tBu ArO)3 tacn)U] ((tBu ArO)3 tacn3– = trianion of 1,4,7-tris(3-tert-butyl-5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) [43, 44].

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Fig. 5 Molecular structure of [(κ 3 -H(R)B(timMe )2 )2 U(THF)3 ][BPh4 ]. Hydrogen atoms omitted for clarity

The 1 H NMR spectrum (benzene-d6 ) at 20 ◦ C displays resonances between –22 and +13 ppm. Two sharp and intense signals at 4.15 and 2.63 ppm are assigned to the tert-butyl groups on the aryloxide pendant arms. The other protons on the frame of the ligand (Ha –Hf ) are diastereotopic. Due to their similar integration values, assignments could not be made. The magnetic moment of solid samples is temperature dependent, varying from 1.77 µB at 5 K to 2.92 µB at 300 K. The low-temperature magnetic moment agrees well with the low-temperature EPR signal, which is a metal-centered isotropic signal with g = 2.005 in an X-band experiment. This complex is highly reactive and crystallization attempts in the presence of methylcyclohexane vapor in the glove box atmosphere produced single crystals of an unprecedented alkane complex [((tBu ArO)3 tacn)U(Me Cy–C6 )], where a C–H bond of methylcyclohexane is bound in an η2 fashion to the electron-rich uranium center [44]. A, respectively. The U–C and U–H distances were found to be 3.864 and 3.192 ˚ This molecule is significant as it is the first fully documented example of stable, crystallographically well-defined metal–alkane coordination. This alkane coordination is a general transformation, as complexes containing cyclohexane, methylcyclopentane, and neohexane were also synthesized and crystallographically characterized. The respective U–O and U–N average distances are ˚. The uranium is situated 0.66 ˚ A below the aryloxide 2.244(3) and 2.676(4) A oxygen atoms, which is slightly less than the value of 0.75 ˚ A obtained for sixcoordinate [((tBu ArO)3 tacn)U]. The difference can be attributed to uranium– ligand bonding interactions, where the alkane has the ability to “pull” the uranium center closer to the plane of the aryloxide oxygen atoms. A simi-

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lar seven-coordinate acetonitrile complex, [((tBu ArO)3 tacn)U(CH3 CN)], has A, indicating that interaca uranium center with an even lower value of 0.45 ˚ tion with π-type ligands causes the uranium center to become even closer to the plane of the oxygen atoms. A second generation of ligands was developed using larger adamantyl groups to prevent undesired reactions, such as bimolecular decomposition and dimer formation. Van der Waals interactions among the adamantyl groups place the reactive U(III) center deep inside the sterically encumA below bering ligand, resulting in the uranium being displaced 0.88 ˚ the plane defined by the three aryloxide oxygen atoms. This compound, [((Ad ArO)3 tacn)U] ((Ad ArO)3 tacn3– = trianion of 1,4,7-tris(3-adamantyl-5tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane), has a protected uranium center with a narrow cylindrical cavity formed by the adamantyl substituents with access to small molecules (Fig. 6) [45]. The 1 H NMR spectrum exhibits the expected 14 paramagnetically shifted and broadened resonances between –22 and 14 ppm. X-ray quality crystals were analyzed and reveal a distorted trigonal prismatic uranium center that is well protected by the bulky adamantyl substituents from the top and the weakly coordinated triazacy˚ clononane ligand from the bottom. The average ligand distances are 2.22(1) A ˚ for U–O and 2.64(3) A for U–N. Variable temperature magnetic data were in the range from 1.74 at 5 K to 2.83 µB at 300 K. The EPR spectrum acquired as a frozen benzene solution at 14 K displayed an isotropic signal at g = 2.005. Replacing the aryloxide functionality with pendant amide arms has been accomplished to make the 1,4,7-tris(dimethylsilylaniline)-1,4,7-triazacyclononane ligand [46]. The synthesis of the brownish-green uranium complex, [((SiMe2 NPh)3 tacn)U], was accomplished by stirring the sodium salt of the ligand with [UI3 (THF)4 ]. Crystallographic analysis revealed a six-coordinate uranium center in a distorted trigonal prismatic conformation, with the two

Fig. 6 Molecular structure of [((Ad ArO)3 tacn)UIII ]. Hydrogen atoms omitted for clarity

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Fig. 7 Molecular structure of [((SiMe2 NPh)3 tacn)U]. Hydrogen atoms omitted for clarity

trigonal planes defined by each set of nitrogen atoms (Fig. 7). The average ˚ is similar to that of 2.320(4) observed U–N(amido) distance of 2.35(2) A in the uranium(III) complex [U(N(SiMe3 )2 )3 ] [39]. The U–N(tacn) distance ˚ is similar to that observed for the previously mentioned arylof 2.66(2) A oxide derivatives, [((R ArO)3 tacn)U] (R=t Bu, Ad) [42]. The paramagnetically broadened and shifted 1 H NMR spectrum revealed a single resonance for the 18 protons of the SiMe2 linkers, two resonances for the six Ho and Hm , and three resonances for Hp protons of the anilide. Three resonances with the integration value of six protons each were also observed (Ha ). Cooling the sample in the NMR probe caused the resonances assigned to the SiMe2 groups and to the methylenic protons to shift, broaden, and collapse. At –60 ◦ C the spectrum sharpened, and four broad resonances in a 1 : 1 : 1 : 1 intensity ratio were visible and assigned to the CH2 groups. The SiMe2 groups also decoalesced at this temperature, and appeared as two peaks in a 9 : 9 intensity ratio, consistent with C3 symmetric structures. The resonances of the aromatic protons of the amido groups did not collapse, indicating fast rotation of the phenyl groups on the NMR timescale at –80 ◦ C.

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Fig. 8 Molecular structure of [(ttcn)UI3 (MeCN)2 ]. Hydrogen atoms omitted for clarity

Substitution of nitrogen for sulfur in the tacn ring allowed isolation of a 1,4,7-trithiacyclononane derivative [47]. Green crystals of the acetonitrile adduct, [(ttcn)UI3 (MeCN)2 ] (ttcn = 1,4,7-trithiaazacyclononane) suitable for X-ray analysis were obtained (Fig. 8). The uranium center is eight-coordinate by three sulfurs of the trithiacrown and can be described as a distorted square ˚ antiprism. The U–S bond distances of 3.0456(9), 3.0146(9), and 3.0779(9) A could indicate a covalent contribution to the U–S bonding. Characterization of this complex by 1 H NMR in acetonitrile solution revealed two resonances integrating to six protons each at 12.67 and 13.65 ppm, presumably due to the ttcn ring.

4 High-Valent Uranium Complexes with Multiply Bonded Ligands 4.1 Complexes Containing the [O = U = O]2+/+ Subunit The most commonly recognized molecular unit in uranium chemistry is no doubt the uranyl ion, [UO2+ ], which has undergone intense study for the past 150 years. The bonding in this linear [UO2 2+ ] unit is quite distinctive, and is made up of a combination of d–p and f –p π interactions. This unit is convenient to work with, because it is stable to moisture and oxygen, and has a convenient handle for infrared spectroscopy, the O=U=O unit. Typically, this band appears from 920 to 980 cm–1 for the asymmetric O–U–O stretch. A band for the symmetric stretch can be viewed by Raman spectroscopy, and

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appears at 860 cm–1 . This measure is important, as the frequency of the symmetric (v1) and asymmetric (v3) UO2 stretch is inversely proportional to the donor strength of the equatorial ligands which lie orthogonal to the [UO2 2+ ] moiety [48, 49]. Electronic absorption spectroscopy typically shows an absorption around 450 nm for the O=U=O unit and is identified by vibrational fine structure typically associated with it [17]. The substitution chemistry of the more common starting materials [UO2 (OTf)2 ] and [UO2 Cl2 ] has been studied. Derivatives of these complexes have been explored by dissolving each in strongly donating solvents or by addition of neutral donor ligands. Exposure to pyridine forms the pyridine adducts [UO2 (OTf)2 (py)3 ] and [UO2 Cl2 (py)3 ]. An X-ray crystallographic analysis performed on the triflate complex shows a neutral monomer in the solid state with monodentate triflate ligands [26]. The analogous complexes, [UO2 (OTf)2 (THF)3 ] and [UO2 (OTf)2 (dme)], were formed by dissolution in the respective solvents. Addition of two equivalents of 2,2′ -bipyridine, three equivalents of phenanthroline, one or two equivalents of terpyridine, or four equivalents of triphenylphosphine oxide (tppo) to [UO2 (OTf)2 (py)3 ] afforded the respective ligand substitution products [UO2 (OTf)2 (bipy)2 ], [UO2 (phen)3 ][OTf]2 , [UO2 (OTf)2 (terpy)], [UO2 (terpy)2 ][OTf]2 , and [UO2 (tppo)4 ][OTf]2 [50]. The uranyl derivatives obtained using 2,2′ -bipyridine and 1,10-phenanthroline show unprecedented rhombohedral coordination geometries around the uranium center [51]. This coordination geometry was also observed for the hydroxide derivative of terpyridine, [{UO2 (OH)(terpy)}2 ][OTf]2 [51, 52]. The triphenylphosphine oxide derivative, [UO2 (tppo)4 ][OTf]2 , is well studied. In attempts to crystallize this uranium(VI) complex, serendipitous crystals of the uranium(V) compound, [UO2 (tppo)4 ][OTf], were obtained along with the uranyl compound [50]. Both were characterized by X-ray crystallography and exhibit a square pyramidal geometry around the uranium center (Fig. 9). The uranium(VI) derivative featured a linear {UO2 } fragment perpendicular to the equatorial plane defined by the uranium center and the four oxygen atoms of the tppo ligands. The mean U=O bond length of ˚ and the average equatorial U–O bond length of 2.29(1) A ˚ are typi1.76(1) A cal [53]. The U(V) complex has slightly longer U=O bond lengths of 1.817(6) ˚ as expected. The U–O(OPPh3 ) bond lengths in the pentavalent and 1.821(6) A ˚ (average 2.44(2) A ˚), again longer complex range from 2.427(5) to 2.455(6) A than in the uranyl derivative, indicating that equatorial bond elongation is favored over elongation of the axial U=O unit. The similar phosphine oxide derivatives [UO2 (tppo)4 ][BF4 ]2 and [UO2 (dppmo)2 (OPPh3 )][X]2 (dppmo = Ph2 P(O)CH2 P(O)Ph2 ) were also prepared from the corresponding uranyl(VI) chloride precursor and two equivalents each of AgX and phosphine oxide [54]. A mixed metal triphenylphosphine oxide derivative, [UO2 (ReO4 )2 (tppo)3 ], was prepared as a monomeric uranyl complex as well. Interestingly, photolytic generation

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Fig. 9 Molecular structure of dication of [UO2 (tppo)4 ][OTf]2 . Hydrogen atoms omitted for clarity

of peroxide in EtOH solutions of this compound forms trace quantities of [((UO2 )(tppo)3 )2 {µ2 –O2 }][ReO4 ]2 , where the coordinated [ReO4 ] groups were displaced by a bridging O2 ligand derived from atmospheric dioxygen [55, 56]. The substitution of the uranyl ion in the presence of biologically relevant neutral donors has produced a class of interesting molecules. For instance, treating [UO2 (NO3 )2 ] with citric, D-(–)-citramalic, or tricarballylic (1,2,3propanetricarboxylic) acids produces two- and three-dimensional frameworks [57–59]. Imidazole coordination has also been explored with the first definitive high-resolution single-crystal X-ray structure for the coordination of 1-methylimidazole (Me imid) to [UO2 (Ac)2 ] (Ac=CH3 CO2 ) (Fig. 10). The

Fig. 10 Molecular structure of [UO2 (Ac)2 (Me imid)2 ]. Hydrogen atoms omitted for clarity

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resulting complex, [UO2 (Ac)2 (Me imid)2 ] [60], features a hexagonal bipyramidal uranium center, with the hexagonal plane occupied by four oxygen and two nitrogen atoms. Further characterization by Raman spectroscopy reveals a stretch at 840 cm–1 for the O=U=O unit. The IR spectrum shows an intense band at 916 cm–1 for the asymmetric uranyl stretch. Infrared spectroscopy also confirms the bidentate coordination mode of the acetate ligands, which have respective symmetric νs (COO) and antisymmetric νa (COO) carboxylate stretching modes at 1468 and 1538 cm–1 . Methylimidazolium uranyl salts have also been reported [61]. Addition of 4,6-O-ethylidene-α-Dglucopyranosylamine with trans-[UO2 2+ ] species produced the corresponding pentagonal bipyramidal product (4) [62].

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The substitution chemistry of [UO2 Cl2 (L)n ]n via salt metathesis has produced aryloxide and iminophosphorane derivatives. The di-tert-butyl phenoxy derivative was synthesized via salt metathesis in THF, producing the dark red [UO2 (O – 2, 6 – tert–Bu2 C6 H3 )2 (THF)2 ] · THF, featuring cisaryloxides and THF molecules [63]. However, when the tert-butyl groups are replaced by phenyl groups, the geometry changes to accommodate transaryloxide groups. Using sterically less bulky chlorine or methyl substituents produces the dimeric products [UO2 (O–2, 6–Cl2 C6 H3 )2 (THF)2 ]2 (one terminal phenoxide, two bridging) and [UO2 Cl(O–2, 6–Me2 C6 H3 )(THF)2 ]2 (one terminal chloride, two bridging phenoxides). The bis-iminophosphorane complexes [UO2 Cl{η3 –CH(Ph2 PNSiMe3 )2 }(THF)] and [UO2 Cl{η3 –N(Ph2 PNSiMe3 )2 }(THF)] were synthesized from the reaction of [UO2 Cl2 (THF)3 ] with Na[CH(Ph2 PNSiMe3 )2 ] and Na[N(Ph2 PNSiMe3 )2 ], respectively [64]. Both are dinuclear complexes in the absence of a coordinating solvent. The crystal structures of both have been determined (Fig. 11), and each display distorted pentagonal bipyramidal geometries with the bis-iminophosphorane ligands coordinating in a tridentate chelating manner. The former complex ˚ in concontains a U–C bond that is out of the equatorial plane by 0.842(3) A trast to the latter complex, where the U–N bond is close to the equatorial ˚). plane (0.154(3) A The first structurally characterized isocyanate actinide derivative, [(UO2 )2 (NCO)5 O]2 [(Et4 N)6 ]·2MeCN·H2 O and isocyanato uranate (Et4 N)6 [(UO2 )2 (NCO)5 O]2 · 2MeCN · H2 O were reported (Fig. 12) [65].Structural

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Fig. 11 Molecular structure of [UO2 Cl{η3 -CH(Ph2 PNSiMe3 )2 }(THF)]. THF and hydrogen atoms omitted for clarity

Fig. 12 Molecular structure of [UO2 (NCO)2 (OP(NMe2 )3 )2 ]. Hydrogen atoms omitted for clarity

characterization revealed that both complexes contain N-bound isocyanate units, similar to [trans-UF4 (NCO)2 ] [66]. In [UO2 (NCO)2 (OP(NMe2 )3 )2 ], the ˚ are significantly shorter than those in d(U–N) bond lengths of 2.336(5) A the corresponding isothiocyanate derivative [UO2 (NCS)2 (OPPh3 )2 ] which ˚ are comparable ˚ [67]. The U=O bond lengths of 1.765(4) A are 2.44(2) A

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with those of other uranyl derivatives. Characterization by infrared spectroscopy reveals a band at 2172 cm–1 assigned to the NCO group as well as one at 911 cm–1 for the O=U=O unit. The isocyanato uranate complex ˚. The terminal U–NCO has uranyl bond lengths of 1.786(6) and 1.795(7) A ˚ A) bond lengths are (d(U–N) = 2.45(1) A) and bridging (d(U–N) = 2.58(1) ˚ significantly longer than the those for the isocyanate compound. Chelating sulfur ligands have been explored with the uranyl unit. For instance, Denning reports the synthesis and characterization of [Ph4 P][UOCl4 (NSPh2 )] from [Ph4 P][UOCl5 ] and the sulfimine ligand, [Ph2 S=NSiMe3 ] (Fig. 13) [68]. In this analogue, one of the trans-uranyl oxygen atoms has been replaced by the sulfimine. The infrared spectrum of this complex shows a stretch at 845 cm–1 , assignable to the antisymmetric O=U=X vibrational mode, which is shifted from that of [UO2 Cl4 ]2– which appears at 922 cm–1 . There is also a band at 1008 cm–1 for the antisymmetric stretch of the U–N–S linkage. The molecular structure of the anion determined by X-ray crystallography shows a pseudooctahedral geometry around the uranium center. ˚) and U–Cl (2.6161(8)–2.6270(8) A ˚) distances are typical The U–O (1.786(3) A ˚ is normal for a U(VI) for a uranyl anion, while the U–N distance of 1.920(3) A imido species. The corresponding phosphoriminato was also synthesized and characterized and the U–O and U–N bond is very similar to the sulfur analogue. Infrared data collected on this compound reveal a symmetric stretch for the O–U–N unit at 863 cm–1 . Bifunctional carbamoyl methyl sulfoxide ligands were treated with [UO2 (NO3 )2 ] to make the corresponding uranyl complexes. The structure of one derivative, [UO2 (NO3 )2 (PhSOCH2 CONi Bu2 )], was determined by a single crystal X-ray diffraction method (Fig. 14) [69]. The uranium atom is

Fig. 13 Molecular structure of the cation of [Ph4 P][UOCl4 (NSPh2 )]. Hydrogen atoms omitted for clarity

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Fig. 14 Molecular structure of [UO2 (NO3 )2 (PhSOCH2 CONi Bu2 )]. Hydrogen atoms omitted for clarity

in a hexagonal bipyramidal geometry ligated by eight oxygen atoms. The bidentate chelating ligand is coordinated through both the sulfoxo and amido oxygen atoms to the uranyl group. The observed bond distance for ˚. ˚, and the U–O(amide) distance is 2.408(9) A U–O(sulfoxide) is 2.442(9) A Although the coordination of carbenes to uranium has been previously explored to uranium(III) [45], the first examples of uranyl–carbon bonds have recently been reported. Treatment of [UO2 Cl2 (THF)3 ] with two equivalents of either 1,3-dimesitylimidazole-2-ylidene (IMes) or 1,3-dimesityl4,5-dichloroimidazole-2-ylidene (IMesCl2 ) produced monomeric uranyl Nheterocyclic carbene complexes [70]. Both complexes were studied by X-ray crystallography, which revealed a near-perfect octahedral uranium center ˚ are (Fig. 15). The respective U=O bond lengths of 1.761(4) and 1.739(3) A in the range of those previously observed for [UO2 Cl2 L2 ] complexes [71– 73]. The U=O bond length for the chloride substituted carbene ligand is significantly shorter, due to the fact that this ligand is a poorer σ donor. ˚, respectively. The carbene The U–C bond lengths are 2.626(7) and 2.609(4) A ligands are oriented so that they avoid steric interaction with the chloride ligands. The U–Cl bond lengths are in the expected range. Analysis of these complexes by infrared spectroscopy showed respective stretches at 938 and 942 cm–1 . These are high in comparison to other derivatives, and thus indicate weak electron donation from the NHC ligands. Soon after this report, the first uranium–methine bond was demonstrated in the centrosymmetric chloro-bridged dimer [UO2 Cl{CH(Ph2 PNSiMe3 )2 }], which consists of two distorted pentagonal bipyramidal units [74]. In the Raman spectrum the

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Fig. 15 Molecular structure of [UO2 Cl2 (IMes)2 ]. Hydrogen atoms omitted for clarity

symmetric O=U=O stretch is observed at 838 cm–1 while the asymmetric stretch is visible in the IR spectrum at 924 cm–1 . The coupling constants calculated from NMR spectroscopy 1 JCX (X=H, P) give insight into the percentage of s character in the C–X bond, in that the larger the value, the more the s character. The coupling constant measured for this complex is 1 JCH = 136.5 Hz, CD2 Cl2 , and is intermediate of the neutral ligand CH2 (Ph2 PNSiMe3 )2 (1 JCH = 123.7 Hz) containing an sp3 carbon and of the ligand precursor [Na{CH(Ph2 PNSiMe3 )2 }] (1 JCH = 144.2) with an sp2 carbon. This is consistent with an interaction between uranyl and the methine carbon atom. The 1 JCP value is less reliable. Electronic absorption data collected on this complex show two strong absorption bands at 515 and 434 nm in CH2 Cl2 . Examination of this compound by X-ray crystallography reveals two distorted pentagonal bipyramidal uranyl units each bridged by two chlorine atoms in a centrosymmetric dimer. Interestingly, the methine unit in this ˚) from the uranyl plane. compound is displaced significantly (0.8877(96) A The author suggests this is due to the filled p orbital, which points toward the uranium center to form an s–p type bond. The U=O bond lengths are ˚. The uranium–carbon distance in the methine unit 1.777(8) and 1.789(8) A ˚, suggesting a U(VI)–C bond, since the sum of the van der Waals is 2.691(8) A A. This is only slightly longer than the U–N bonds of 2.514(7) and radii is 3.56 ˚ ˚. Functionalization of an N-heterocyclic carbene with a pendant 2.458(7) A amine arm afforded a uranyl amido-NHC derivative, [UO2 L2 ] (L=1-ethylenetert-butylamino-2-R-imidazol-2-ylidene, R=t Bu) [75]. This molecule features distortion of the U–C bond from the expected trigonal planar hybridization. A and the bent geometry of the carThe carbene–uranium distance is 2.64 ˚ bene ligand indicates that this interaction is electrostatic. Characterization by

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FTIR showed a spectrum with a peak at 929 cm–1 assigned to the asymmetric stretch for the O=U=O unit. Similar work has been performed with an alkoxide functionality [76]. The chemistry of uranyl units with nitrogen-containing ligands, such as Schiff bases and salen ligands (L), has been explored. trans-Dioxouranium dinuclear complexes of OH-containing ligands with N-, O-coordination sites were synthesized and characterized [77]. Seven of these were also structurally characterized by single crystal X-ray diffraction (Fig. 16). All of these complexes exhibit symmetric [U2 O2 ] core structures with a seven-coordinate uranium center. Ligands with more than one CH2 OH group only had one involved in chelation and in bridging. Despite the similarity of their molecular structures, their lattice arrangements display novel types of structures such as channel, herringbone, and corrugated sheets from extended weak interactions. Characterization of the 11 reported complexes by infrared spectroscopy revealed a band in the range 897–912 cm–1 assignable to asymmetric stretching of the trans-dioxouranyl ion. The ν(CN) vibrations in the region 1610–1629 cm–1 are shifted by at least 10–20 cm–1 to lower frequency as compared to those of the corresponding “free” ligands, indicating that the nitrogen of azomethine is coordinated to the metal center. In several cases this band for the Schiff base ligand disappears due to the reduction of the imine by the metal center; hydroxyl groups are observed in the expected region. Another example explores solvated derivatives of the type [UO2 (salophen)L] (salophen = N,N ′ -disalicylidene-o-phenylenediaminate, L = DMF, DMSO),

Fig. 16 Molecular structure of [UO2 (5–Br–H2 L(DMF)]. Dimethylformamide molecules and hydrogen atoms omitted for clarity

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as well as the unsolvated version [78]. The unsolvated version is a racemic dimeric complex, [UO2 (salophen)]2 , which undergoes equilibrium with the solvated complex. Another study reports the first example of UO2 + –UO2 + interaction and unambiguous evidence of the presence of the resulting tetrameric cation– cation complex in pyridine solution [79]. Reaction of coordination polymer [(UO2 (py)5 )(KI2 (py)2 )]n with two equivalents of Kdbm (dbm– = dibenzoylmethanate) in pyridine allows the isolation, after diffusion of diisopropyl ether, of blue crystals of the tetrameric pentavalent uranium complex [UO2 (dbm)2 ]4 [K6 py10 ] · I2 · 2py in which four [UO2 (dbm)2 ] complexes are assembled by cation–cation interactions between the [UO2 + ] units (Fig. 17). Analysis of the crystals by X-ray crystallography revealed a centrosymmetric tetramer of [UO2 + ] units coordinated to each other in a monodentate fashion to form a square plane with two crystallographically inequivalent uranium units. Each [UO2 + ] coordinates two adjacent groups and is involved in two T-shaped cation–cation interactions (two linear [UO2 + ] groups arranged perpendicular to each other). Each [UO2 + ] is also involved in a cation–cation interaction with a potassium ion. The [UO2 + ] groups have U–O distances of ˚, and are much shorter than those observed for the 1.923(10) and 1.934(8) A ˚ dbm oxygens (2.44(2) A). However, the uranium distance is very similar to

Fig. 17 Molecular structure of [UO2 (dbm)2 ]4 [K6 py10 ] · I2 · 2py. Hydrogen, carbon, and iodine atoms omitted for clarity

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that of the starting material, indicating that the uranium(V) oxidation state is preserved. The asymmetric stretch for the O=U=O units by infrared spectroscopy is found at 782 cm–1 . These results are significant as they expand the possibilities for the preparation of polymetallic assemblies involving f elements [80]. In addition, they offer the possibility for reaction of the pentavalent O=U=O fragment with other metals, including 3d transition and other actinide metals. 4.2 High-Valent Uranium Complexes with Nitrogen Donor Ligands Some studies have examined the formation of high-valent uranium complexes which do not contain a uranyl unit. Hexakisamido uranium complexes have recently been explored by using the sterically bulky amine precursor Hdbabh (Hdbabh = 2,3 : 5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5diene) [81]. Addition of seven equivalents of the corresponding lithium salt to [UI3 (THF)4 ] produced an orange solid assigned as [Li(THF)x ][U(dbabh)6 ]. EPR in frozen acetonitrile/toluene solution displayed a single isotropic resonance at g = 1.12, similar to other uranium(V) complexes. The magnetic moment of the n Bu4 N+ salt is 1.16 µB between 5 and 35 K and increases to 3.7 µB at room temperature, supporting the U(V) formulation. Electrochemical studies confirmed an oxidation wave at –1.10 V. X-ray crystallography showed a near perfect octahedral complex, with U–N distances ranging from ˚. DFT calculations indicate that the unpaired elec2.230(11) to 2.267(13) A tron resides in an f (xyz) orbital. Oxidation by air or silver nitrate produces the neutral U(VI) complex, [U(dbabh)6 ] (Fig. 18). Crystallographic analyses of both uranium complexes revealed near perfect octahedral coordination, ˚ for with typical U–N bond distances ranging from 2.230(11) to 2.267(13) A ˚ the U(V) compound and 2.178(6) to 2.208(5) A for U(VI). The electronic absorption spectrum of the uranium(VI) derivative has two distinct bands at 353 nm (ε = 2200 M–1 cm–1 ) and 501 nm (ε = 1200 M–1 cm–1 ). DFT confirms that the neutral molecule features amido to uranium π bonding. The HOMOs are mostly nitrogen 2p in character, with contributions from U 5f and 6d. This chemistry is unique because both the U(V) and U(VI) analogues can be studied with minimal structural change. Cummins has described the synthesis of bimetallic µ-cyanoimide complexes made from an [NCN] transfer reagent, the cyanoimide based on Hdbabh [82]. Treating [(Ar[R]N)3 U(THF)] with 0.5 equivalents of NCdbabh produced the corresponding µ-cyanoimide complex [((Ar[R]N)3 U)2 {µ2 , η1 , η1 -NCN}] (Ar=C6 H3 Me2 -3,5; R=t Bu). X-ray crystallography revealed a bent geometry at the cyanoimide nitrogens, with a U–N–C angle of 162.6(5)◦ (Fig. 19). The bent geometry of the cyanoimide nitrogens in (µ-NCN) and the similarity of the U–Namide and U–Ncyanoimide distances indicate that there is little π bonding in the uranium–cyanoimide interaction. The isotopomer

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Fig. 18 Molecular structure of [U(dbabh)6 ]. Hydrogen atoms omitted for clarity

Fig. 19 Molecular structure of [((Ar[R]N)3 U)2 {µ2 , η1 , η1 -NCN}]. Hydrogen atoms omitted for clarity

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made by using the 13 C-labeled reagent N13 Cdbabh displays a broad 13 C NMR resonance at 133 ppm assigned to the central carbon in the µ-cyanoimide unit [82]. While uranium(V) imido species have previously been synthesized, these compounds are metallocene based [83–85]. Recently, non-cyclopentadienyl ancillary ligands have been shown to support uranium(V) imido complexes as well. For instance, the triamidoamine ligand scaffold [NN3 ′ ] protects a trimethylsilyl uranium imido species [86]. Uranium(V) imido complexes based on the aryloxide substituted tacn system have also been explored. Addition of an equivalent of trimethylsilyl azide to either [((tBu ArO)3 tacn)U] or [((Ad ArO)3 tacn)U] in benzene forms the high-valent uranium(V) imido compounds, [((R ArO)3 tacn)UV (NSiMe3 )] (R=tBu, = Ad) [44, 87]. Variable temperature magnetic data collected for the tert-butyl derivative show a magnetic moment of µeff = 2.34 µB at 300 K that lowers to µeff = 1.46 µB at 5 K. The room temperature moment is smaller than the expected value of 2.54 µB calculated for the free ion in the L–S coupling scheme due to increased covalency in the bonding interactions. Electronic absorption spectroscopy shows an intense charge-transfer band at 400 nm (ε = 3800 M–1 cm–1 ). High-valent uranium(V) and (VI) imido species typically exhibit short, formal U–N(imido) A and  (U–N–R) triple bonds with bond distances ranging from 1.85 to 2.01 ˚ bond angles varying from slightly bent to linear (163.33–180.0◦ ). Accordingly, the structural parameters of the tert-butyl derivative (d(U–N(imido)) ˚ and  (U–N–R) = 173.7(3)◦ ) are similar. DFT calculations sup= 1.989(5) A port the formulation of the U–N bond as a formal triple bond, containing two π-bonding and one σ-bonding interactions. The more sterically hindered adamantyl compound (Fig. 20) exhibits a U–N(imido) bond distance that is the longest ever reported for a metal imido complex, and deviates

Fig. 20 Molecular structure of [((Ad ArO)3 tacn)UV (NSiMe3 )]. Hydrogen atoms omitted for clarity

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Fig. 21 Molecular structure of [((Ad ArO)3 tacn)U(NCNMe)]. Hydrogen atoms omitted for clarity

˚ and  (U–N–R) = 162.55(12)◦ ). from linearity (d(U–N(imido)) = 2.1219(18) A This conformation is most likely due to the steric pressure imparted by the adamantyl groups as they form a narrow cylindrical cavity and prevent the Me3 SiN2– ligand from optimal binding. Accordingly, the imido nitrogen p orbitals cannot participate in efficient M–L π bonding, resulting in the long U–N bond and increased reactivity. Addition of π acids such as carbon monoxide or methyl isocyanide to [((Ad ArO)3 tacn)UV (NSiMe3 )] resulted in formation of the respective uranium(IV) isocyanate and carbodiimide complexes, [((Ad ArO)3 tacn)U(η1 -NCO)] and [((Ad ArO)3 tacn)U(η1 -NCNMe)], with loss of Me6 Si2 (Fig. 21). The respective IR spectra show strong bands at 2185 and 2101 cm–1 assigned to the η1 -coordinate isocyanate and carbodiimide ligand. The U–N bond lengths and  U–N4–C70 angles were determined to ˚ and ˚ and 171.2(6)8 ◦ for the isocyanate complex and 2.327(3) A be 2.389(6) A ◦ 161.9(3)8 for the carbodiimide derivative. Magnetic and electronic absorption data are consistent with the uranium(IV) formulation. The UV/vis/NIR spectra of both colorless complexes are similar with various sharp, low-intensity bands (ε = 5–80 M–1 cm–1 ), which are characteristic for spectra of U(IV), f 2 complexes with a 3 H4 ground state. This additional reactivity is due to the bend in the imido fragment, which imparts additional nucleophilic character. These transformations are believed to occur through multiple bond metathesis with π acids [27]. 4.3 Unprecedented Uranium(IV) Coordination Complexes The uranium(IV) complex [(C–N–C)UCl4 (THF)] with the “pincer” 2,6bis(imidazolylidene)pyridine [88] ((C–N–C) = 2,6-bis(arylimidazol-2-ylid-

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ene)pyridine, aryl = 2,6-Pri2 C6 H3 ) has been synthesized, and is only the second U(IV) N-heterocyclic carbene complex (Fig. 22). The first is the metallocene-based compound, [Cp∗ 2 U(O)(C(MeNCMe)2 )] [89]. The uranium in [(C–N–C)UCl4 (THF)] has a distorted seven-coordinate geometry with an approximate C2 axis passing through the pyridine N atom and the A. The U–C uranium. The U–Cl bond distances range from 2.587 to 2.673 ˚ ˚) are shorter than those observed carbene bond lengths (2.573(5)–2.587(5) A ˚), U(III) (2.672(5)–2.789(14) A ˚) [45, 75], and previously for U(IV) (2.636(9) A A) [70]. However, they are longer than other known U(VI) complexes (2.64 ˚ A) [90–92]. U–C σ(sp3 )-alkyl bonds (2.405–2.539 ˚ The high-yield synthesis and spectroscopic and structural characterization of a dimeric uranium(IV) halide complex [{[t BuNON]UCl2 }2 ] supported by the doubly deprotonated diamidosilyl ether ligand [((CH3 )3 CNH(Si(CH3 )2 ))2 O]([t BuNON]2 ) are reported (Fig. 23) [93]. The –C(CH3 )3 protons are assigned to the singlet at δ 68.9. Two broad upfield peaks at δ –17.7 and –23.8 correspond to the –Si(CH3 )2 groups. The presence of two resonances for the –Si(CH3 )2 substituents is consistent with the dimeric nature of the complex in toluene-d8 . A variable temperature NMR study between 293 and 353 K showed that the two resonances became broader as the temperature increased, coalescing at 353 K. Either the rapid interconversion of the bridging and terminal chlorides or a monomer–dimer equilibrium could yield equivalent ligand silyl methyl moieties. The solid-state structure of the dimer, with ˚. The partial occupancy of Br for Cl, showed a U–O distance of 2.479(11) A

Fig. 22 Molecular structure of [(C–N–C)UCl4 (THF)]. Hydrogen atoms and THF omitted for clarity

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Fig. 23 Molecular structure of [{[t BuNON]UCl2 }2 ]. Hydrogen atoms omitted for clarity

U–Br has chloride/bromide disorder in the structure, precluding a meaningful discussion of structural parameters. Variable temperature magnetic data were recorded and produced a µeff value of 2.63 µB at 300 K which decreases to 0.81 µB at 2 K per uranium center. The author states that the change in µeff values at low T may be partially attributed to weak antiferromagnetic interactions between the two uranium(IV) centers, mediated by the chloride bridges. However, complexes of U(IV) with 3 H4 ground states typically show magnetic moments around 0.5–0.8 µB. Addition of alkylating agents to the halide precursor produced both the η1 -bis(allyl) and bis(alkyl) species. The η1 coordination mode of the allyl species was confirmed by both the 1 H NMR spectrum (294 K), showing broad resonances of δ 29.7 and 11.4 for CH(CH2 )2 , as well as the the IR spectrum, which has a stretch at 1617 cm–1 typically absent in the η3 -coordinated species. For the bis(alkyl) compound, [{tBu NON}U(CH2 Si(CH3 )3 )2 ], the 1 H NMR spectrum displays sharp resonances which are paramagnetically shifted. The protons on the α carbon are shifted significantly upfield to δ –148.9 due to their proximity to the uranium center. The first non-metallocene uranium silyl compound, [(Ar[t Bu]N)3 USi(SiMe3 )3 ] (Ar = 3,5-C6 H3 Me2 ), has been synthesized using the tris(N-tertbutylanilide) ligand scaffold (Fig. 24) [94]. Addition of (THF)3 LiSi(SiMe3 )3 to [(N[t Bu]Ar)3 UI] (Ar=3, 5–C6 H3 Me2 ) (diethyl ether, –100 to 25 ◦ C, 10.5 h) afforded a red solid in ca. 80% yield after filtration, concentration, and recrystallization. Crystallographic and computational studies were performed to elucidate the bonding. This unique uranium silyl compound has a U–Si

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Fig. 24 Molecular structure of [(Ar[t Bu]N)3 USi(SiMe3 )3 ]. Hydrogen atoms omitted for clarity

˚; however, no other molecular U–Si distances have bond distance of 3.091(3) A been reported in the literature for comparison. Geometry optimizations were carried out for the model systems H3 EU(NH2 )3 (E=C, Si, Ge, Sn) with a set of reasonable constraints; calculated U–E distances and bond energies are in accord with experimental data obtained for both [(Ar[t Bu]N)3 USi(SiMe3 )3 ] and [(Ar[t Bu]N)3 UMe]. There is a slight disparity of the calculated and experimental bond distances for U–Si, which may signify a stressed U–Si contact due to the bending of the SiMe3 groups away from the bulky anilide ligand. ˚) are typical for a uranium(IV) derivative supThe U–N distances (2.210(5) A ported by the N-tert-butylanilide ligand [30, 95] and compare well with the A calculated value. 2.230 ˚ Ephritikhine reports the first oxalamidino compound of a 5f element. Addition of {Li(THF)}2 (m-C2 N4 R4 ) (R=Cy) to UCl4 produced [Li(py)4 ]2 [(UCl4 (py))2 {µ-C2 N4 R4 }] (R=Cy) [96]. The crystal structure of the dark green pyridine adduct was determined, and revealed a binary axis containing the two uranium atoms, the two central carbon atoms of the diamidinate ligand, and the nitrogen atoms of the coordinated pyridine molecules (Fig. 25). The two (CyN)2 C fragments of the bridging tetradentate ligand are nearly perpendicular to one another, the dihedral angles between the two UN2 C mean planes being 89.9(4) and 88.2(4)◦ in the two anions, respectively, to minimize the interactions between the cyclohexyl groups. The uranium

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Fig. 25 Molecular structure of [Li(py)4 ]2 [(UCl4 (py))2 {µ-C2 N4 R4 }]. Lithium cation and hydrogen atoms omitted for clarity

atoms are seven coordinate in a distorted pentagonal bipyramidal configuration, in which two chlorine atoms and the three nitrogen atoms define the basal plane and the other two chlorine atoms are in apical positions. The ˚, reU–N(py) and U–Cl bond lengths, which average 2.68(3) and 2.66(2) A spectively, are unexceptional for U(IV) complexes and may be compared with ˚ in [UCl4 (py)4 ]. The mean U–N(oxalamidino) those of 2.702(1) and 2.638(4) A ˚ bond length is 2.417(7) A. The complex [U{N(SiMe3 )2 }2 {N(SiMe3 )(SiMe2 CH2 B(C6 F5 )3 )}] is formed by hydrogen evolution in the reaction between the hydride complex [U(N(SiMe3 )2 )3 (H)] and B(C6 F5 )3 . The X-ray and neutron structures have been determined and show an electron-deficient uranium center capable of forming multicenter bonds between U and three Si–CH2 units of the amido ligands. The similar complex [U(C(Ph)(NSiMe3 )2 )2 {µ3 -BH4 }2 ] was analyzed as well, and the X-ray structure proves unequivocally the η3 coordination of the BH4 moieties. In both single-crystal structure determinations, all hydrogen and deuterium atoms could be located and isotropically refined, including those which are directly coordinated to the uranium (Fig. 26). The ability to locate the hydrogen and deuterium positions in these uranium compounds by single-crystal X-ray diffraction is due to good crystal quality, the measurement of data at low temperature, and the use of image plate technology for data collection [97]. The first organometallic dication of an f element was recently reported. Treating [(COT)U(BH4 )(HMPA)3 ][BPh4 ] with NEt3 HBPh4 gave [(COT) U(HMPA)3 ][BPh4 ]2 (HMPA = hexamethylphosphoramide). The crystal structure of the pyridine solvate shows that the dications adopt a three-legged piano-stool configuration in which the O–U–O angles have a mean value of 87(3)◦ , and the COT–U–O angles centroid of the C8 H8 ring range between

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Fig. 26 Molecular structure of [U(C(Ph)(NSiMe3 )2 )2 {µ3 -BH4 }2 ]. Hydrogen atoms omitted for clarity

˚ from 127.2 and 128.7◦ , averaging 127(1)◦ . The uranium atom is 1.92(2) A ˚ the planar cyclooctatetraene ring (within 0.01 A), and the mean U–C bond ˚ (Fig. 27). distance is 2.65(3) A

Fig. 27 Molecular structure of the cation of [(COT)U(HMPA)3][BPh4 ]2 . Anion and hydrogen atoms omitted for clarity

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5 Uranium Coordination Complexes with Chalcogen-Containing Ligands Although there have been extensive studies of the coordination chemistry of uranium with nitrogen-based ligands, far less work has been done using softer chalcogenide-based ones. Uranium is an ideal choice for reactivity with these elements because of its extreme oxophilicity and high reduction potential. Industrially, these complexes are attractive due to their potential use as nuclear fuels [98]. In addition, sulfur-based ligands are used in nuclear waste management for lanthanide(III)/actinide(III) differentiation [99]. The complexes described here show the variety of oxidation states and coordination environments accessible to these elements. The first uranium(IV) dithiolene complex, [Na(18-crown-6)]2 [(COT)U (dddt)2 ], was synthesized by treating [(COT)UX2 (THF)n ] (X=BH4 and n = 0; X = I and n = 2) with [Na2 (dddt)] (dddt = 5,6-dihydro-1,4-dithiin-2,3dithiolate) (Fig. 28). Analysis of the red crystals revealed an average U–S ˚ and an exo–exo conformation of the two dithiolate bond length of 2.7782(6) A ligands. Oxidation with Ag+ gave the black uranium(V) complex [Na(18crown-6)(THF)][(COT)U(dddt)2 ], whose crystal structure revealed an exo– ˚ endo conformation of the dddt ligands and a mean U–S distance of 2.693(5) A (Fig. 28) [100, 101]. The uranium center has a distorted square pyramidal ˚ above the basal plane formed by the four geometry and lies 1.286(1) A ˚) dis˚) and C=C (1.357(9) and 1.363(9) A S atoms. The C–S (average 1.75(1) A tances indicate that there is little electron delocalization on the dithiolene ligands. The unsolvated derivative, [Na(18-crown-6)][(COT)U(dddt)2 ], was also prepared, and assigned as uranium(V) based on its characteristic dark purple color.

Fig. 28 Molecular structures of the anions of [Na(18-crown-6)(THF)][(COT)UV (dddt)2 ] (left) and [Na(18-crown-6)]2 [(COT)UIV (dddt)2 ] (right). Cations and hydrogen atoms omitted for clarity

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The difference in the ligand conformation between the U(V) and the U(IV) ddt derivatives was explored extensively by DFT, which confirmed that the oxidation state was responsible for the difference in the conformation of the ligand [101]. The calculations also confirm a significant U–(C=C) interaction between the metal center and the C=C bond of the endo dithiolene ligand in the uranium(V) complex, which does not exist in the dianionic uranium(IV) species. A metal f ligand back-donation occurs in both complexes from the partially occupied uranium 5f orbitals toward the vacant π∗ (C=C) antibonding MO of the dithiolene ligand, which becomes partially occupied after interaction. Subsequent to this initial report, the neutral derivative of [Na(18-crown6)]2 [(COT)U(dddt)2 ] was synthesized by reaction of dddtCO with the same starting material to produce the green [(COT)U(dddt)2 ] dimer. A similar black derivative, [(COT)U(dmio)]2 (dmio = 1,3-dithiole-2-one-4,5-dithiolate), was also synthesized and decarboxylated by BH3 ∗ Me2 S to produce the neutral [(COT)U(mdt)]2 (mdt = 1,3-dithiole-4,5-dithiolate). This complex (Fig. 29) as well as its pyridine derivative (Fig. 30) were crystallographically characterized [102]. The parent complex, [(COT)U(mdt)]2 , exists as an unsymmetric ˚ are 0.13 ˚ A dimer, where the U–S bond lengths of 2.810(2) and 2.816(2) A shorter than those on the other side of the dimer. Also, the S2 C2 S2 core of the mdt ligand is outside the bisecting plane of U–U and forms a dihedral angle of 60◦ . This deviation corresponds to a folding of the US2 C2 ring by 81.9(2)◦ along the S–S axis, bringing the C–C atoms in proximity to the uranium atom, and causes the deviation of the U2 S4 C4 core from D2h symmetry. The U–C ˚, but the C=C fragment shows no bond lengths are 2.950(8) and 2.948(8) A

Fig. 29 Molecular structure of [(COT)U(mdt)]2. Hydrogen atoms omitted for clarity

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Fig. 30 Molecular structure of [(COT)U(mdt)(py)2]. Hydrogen atoms omitted for clarity

elongation from reduction with the uranium center. The molecular structure of the pyridine derivative, [(COT)U(mdt)(py)2 ], displayed a monomer with an η4 coordination mode of the mdt ligand. The U–S distances of 2.720(3) and ˚ are ca. 0.08 ˚ A shorter than in the parent dimer. The interaction of 2.751(3) A the C=C bond and the uranium center is observed in the pyridine adduct as ˚. The folding dihedral angle well. The U–C distances are 2.89(1) and 2.97(1) A ◦ of the mdt group (75.6(3) ) is also similar to the parent, but in this case the dithiolene ligand has an endo conformation. The solvated derivative shows fluxional behavior by 1 H NMR spectroscopy. The first tris- and tetrakis(dithiolene) complexes have also been synthesized and characterized [103]. Treating UCl4 with 3 or 4 mol equiv of Na2 dddt (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) in THF afforded the first example of a homoleptic tetrakis(dithiolene) metal compound, [Na4 (THF)8 U(dddt)4 ] (Fig. 31). The complex was characterized by a singlet at δ 5.98 (pyridine-d6 ) in the 1 H NMR spectrum, consistent with magnetically equivalent dddt ligands due to rapid exchange of the dithiolene ligands and sodium ions on the NMR timescale. Red crystals of [Na4 (THF)8 U(dddt)4 ] were grown in THF, and X-ray diffraction analysis revealed that the crystals are composed of infinite chains in which each U(dddt)4 unit is surrounded by four Na atoms, two of those being involved in bridging Na2 (µ-THF)3 fragments; the uranium atom is eight coordinate and has a dodecahe˚, dral geometry. The U–S distances vary from 2.7900(19) to 2.8654(18) A ˚ with an average value of 2.83(3) A. Treatment of UCl4 with 3 mol equiv of

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Fig. 31 Molecular structure of [Na4 (THF)8 U(dddt)4 ]. Hydrogen atoms omitted for clarity

Na2 dddt in pyridine gave a mixture of tris- and tetrakis(dithiolene) compounds. Analysis by 1 H NMR spectroscopy of a pyridine solution showed the tris(dithiolene) compound, [Na2 (py)x U(dddt)3 ], as a resonance at δ –2.84. After addition of 18-crown-6, only the tris(dithiolene) complex was obtained as orange crystals of [Na(18-crown-6)(py)2 ]2 [U(dddt)3 ] · 2py, in which the isolated [U(dddt)3 ]2– anion adopts a slightly distorted trigonal prismatic configuration. A few red crystals of the trinuclear anionic compound [Na(18crown-6)(py)2 ]3 [Na{U(dddt)3 }2 ] were also obtained. Both tris(dithiolene) compounds exhibit large folding of the dddt ligand and significant interaction between the C=C double bond and the metal center. The first neutral uranium thiolate compounds have also been studied [104]. Reaction of [U(NEt2 )4 ] with HS-2,4,6-t Bu3 C6 H2 (HSMes∗ ) afforded [(SMes∗ )3 U(NEt2 )(py)], while similar treatment of [U(N(SiMe3 )SiMe2 CH2 ) (N(SiMe3 )2 )2 ] produced [(SMes∗ )U(N(SiMe3 )2 )3 ]. The solid-state structure of [(SMes∗ )3 U(NEt2 )(py)] revealed a distorted trigonal bipyramidal configu˚ from the basal plane of the ration where the uranium atom lies 0.3549(7) A ˚ (Fig. 32). This three sulfur atoms with an average U–S distance of 2.695(18) A molecule features a U–H–C β agostic interaction from the coordinated amide. The molecular structure of red crystals of [(SMes∗ )U(N(SiMe3 )2 )3 ] show U–S ˚. In this case, there is a γ U–H–C interdistances of 2.6596(8) and 2.696(3) A action from one of the methyls of the trimethylsilyl substitutent. Homoleptic

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Fig. 32 Molecular structure of [(SMes∗ )U(N(SiMe3 )2 )3 ]. Hydrogen atoms omitted for clarity

[U(SMes∗ )4 ] can be isolated from the reaction of [U(BH4 )4 ] and KSMes∗ as a black powder. In the molecular structure, the U–S distances of 2.6173(9) and ˚ are similar to those previously observed. There is an agostic inter2.6294(9) A action between the C–H bond of one of the tert-butyl groups and the metal center. The first homoleptic thiolate complex of uranium(III), [U(SMes∗ )3 ], was synthesized by protonolysis of [U(N(SiMe3 )2 )3 ] with HSMes∗ in cyclohexane to produce dark brown crystals. The crystal structure exhibits the novel η3 ligation mode for the arylthiolate ligand, and an average U–S dis˚. The distance between the trigonal pyramidal uranium tance of 2.720(5) A and the carbon atoms involved in the U–H–C agostic interaction of each thiA, than that expected from a purely ionic olate ligand is shorter, by ∼0.05 ˚ bonding model. DFT reveals that the nature of the U–S bond is ionic and strongly polarized at the sulfur for uranium. The strength of the U–H–C agostic interaction is believed to be controlled by the maximization of the interaction between Uδ+ and Sδ– under steric constraints. The η3 ligation mode of the arylthiolate ligand is also obtained from DFT. The uranium–chalcogenide complexes, U[N(EPPh2 )2 ]3 (E=S or Se), were synthesized by treating [U(N(SiMe3 )2 )3 ] with three equivalents of the neutral ligand NH(EPPh2 )2 (5) [105]. The electronic absorption spectra of both complexes were acquired as benzene solutions, and display U(III) Laporteforbidden 5f –5f transitions with weak absorption bands (750–1300 nm) in the near-IR region and more intense bands (550–700 nm) in the visible region, assigned as Laporte-allowed 5f –6d transitions [10]. An intense charge-

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transfer band can be found below 400 nm. Analysis by infrared spectroscopy shows P–E vibrations at 593 cm–1 (E=S) and 536 cm–1 (E=Se). Both are lower in energy than the corresponding free ligands, indicating deprotonation and coordination to the uranium center. The structures of both complexes were determined by X-ray diffraction, and showed a nine-coordinate U(III) center in a distorted tricapped trigonal prismatic coordination environment. Each uranium is bound to three [N(EPPh2 )2 ]2 anions. In the sulfur compound, the ˚, the U–N distance is 2.632(2) A ˚, the S–U–S bite U–S distance is 2.9956(5) A ◦ ◦ angle is 122.82(2) , and the P–N–P angle is 147.43(16) . The U–Se distance ˚, ˚, the U–N distance is 2.701(3) A in the selenium compound is 3.0869(4) A the Se–U–Se bite angle is 124.594(12)◦ , and the P–N–P angle is 144.5(2)◦ . Because of the steric demands of the ligand, only three molecules fit around the uranium center, dictating the trivalent oxidation state.

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The tellurium analogues of these ligands have been prepared in a similar manner, but use stabilizing i Pr substitutents instead of phenyl groups. Treating [UI3 (py)4 ] with three equivalents of [Na(tmeda){N(TePi Pr2 )2 }] produced [U(N(TePi Pr2 )2 -Te,Te′ )3 ] as a blue-gray solid (Fig. 33) [106]. Unlike the sulfur and selenium analogues, in the case of the tellurium ligand the central nitrogen atom of the ring is not coordinated (U–N: ∼5 ˚ A), creating a six-coordinate U(III) center in a distorted trigonal prism. This dichotomy in ligand coordination is attributed to the increased steric demand of the i Pr substituents and the larger size of the tellurium atom. The average U–Te ˚ and the average Te–U–Te bite angle is 91.01(3)8◦ . The distance is 3.164(2) A UV/vis/NIR spectrum displays absorption bands between 480 and 1300 nm (ε = 380–1269 M–1 cm–1 ) due to Laporte-forbidden 5f –5f transitions and allowed 5f –6d transitions. Treatment of uranium metal with PhEEPh (E=S, Se) in the presence of a catalytic amount of iodine in pyridine affords the monomeric, sevencoordinate U(IV) chalcogenolates, [U(EPh)4 (py)3 ], which do not require stabilizing ancillary ligands [107]. Crystallographic characterization of the sulfur derivative showed a distorted pentagonal bipyramidal uranium cen˚, and an average U–N distance ter, an average U–S distance of 2.734(3) A ˚ of 2.597(9) A. Spectroscopic comparison of the sulfur and selenium com-

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Fig. 33 Molecular structure of [U(N(TePi Pr2 )2 -Te,Te′ )3 ]. Hydrogen atoms omitted for clarity

pounds suggests that the selenium compound is of similar structure. The dimeric eight-coordinate complexes [U(EPh)2 (µ2 -EPh)2 (CH3 CN)2 ]2 are obtained by crystallization from solutions of the pyridine complexes dissolved in acetonitrile (Fig. 34). For these complexes, the molecular structures show a uranium center in a triangular dodecahedral geometry. For the sulfur ˚ while the U–η2 S distances of derivative, the U–η1 S distance is 2.813(2) A ˚ 2.9378(19) and 2.8667(19) A are longer. The selenium analogue had a U– ˚ and longer U–η2 Se distances of 3.0564(10) and η1 Se distance of 2.8491(12) A ˚. Oxidation of U(0) by pySSpy (C5 H5 NSSC5 H5 N) and crystalliza2.9935(11) A tion produced a nine-coordinate compound, U(Spy)4 (THF), as a distorted tricapped trigonal prism, which had average U–S and U–N distances of ˚, respectively. Formation of the distorted cubane 2.8299(16) and 2.540(4) A cluster [U(py)2 (SePh)(µ3 -Se)(µ2 -SePh)]4 · 4py by addition of elemental selenium and diphenyldiselenide produced complexes in which each U(IV) ion is eight-coordinate and the U4 Se4 core. In this structure, the U–η1 SePh ˚, ˚, the average U–η2 SePh distance is 3.0614(14) A distance is 2.9267(10) A 2– ˚, and the average U–N disthe average U–Se distance is 2.8681(15) A ˚. The electronic absorption spectra of [U(EPh)4 (py)3 ], tance is 2.619(10) A [U(Spy)4 (THF)], and [U(py)2 (SePh)(µ3 -Se)(µ2 -SePh)]4 · 4py display bands arising from f –f and f –d transitions. Distinctive bands appear in pyridine between 685 and 687 nm and 1165 and 1170 nm for [U(EPh)4 (py)3 ] and [U(py)2 (SePh)(µ3 -Se)(µ2 -SePh)]4 · 4py which are indicative of U(IV). The UV/vis/NIR spectrum of [U(Spy)4 (THF)] in benzene has absorptions at 699,

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Fig. 34 Molecular structure of [U(SePh)2 (µ2 -SePh)2 (CH3 CN)2 ]2 . Hydrogen atoms omitted for clarity

1130, and 1215 nm. The poor solubility of [U(EPh)2 (µ2 -EPh)2 (CH3 CN)2 ]2 precluded analysis in benzene, but a UV/vis/NIR spectrum of the compound generated in situ revealed characteristic absorbances at 687 and 1125 nm. All of these complexes also show intense charge-transfer bands below 500 nm. No extinction coefficients were reported. The first example of a homoleptic actinide complex containing threemembered rings is a thermally stable η2 -sulfenamido complex of uranium, [U(η2 -t BuNSPh)4 ] [108] (Fig. 35). This complex was synthesized by a salt metathesis reaction of LiN(t Bu)SPh with UCl4 /PMe3 and isolated as airsensitive yellow-brown crystals that produce a paramagnetically shifted and broadened 1 H NMR spectrum. A diagram of the possible resonance structures for the η2 forms A (sulfenamide) and B (iminosulfide, N-alkylsulfidimino) is presented below. The molecular structure was determined by X-ray diffraction and confirms the η2 bonding mode to the uranium center, similar to the bonding of the sulfenamido ligand with the early transition metals Ti, Zr, Mo, A, A and U–S distance is 2.87 ˚ and W [109]. The average U–N distance is 2.30 ˚ similar to the previously discussed chalcogenide compounds. Reaction of UX4 (X=Cl, BH4 ) and two equivalents of [Li(Et2 O)][SPSMe ], the lithium salt of an anionic SPS pincer ligand composed of a central hypervalent λ4 -phosphinine ring bearing two ortho-positioned diphenylphosphine sulfide side arms, formed complexes of the type [UX2 (SPSMe )2 ] (Fig. 36). Crystals of the chloride derivative were obtained as the pyridine adduct, and

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Fig. 35 Molecular structure of [U(η2 -t BuNSPh)4 ]. Hydrogen atoms omitted for clarity

Fig. 36 Molecular structure of [UCl2 (SPSMe )2 ]. Phenyl substituents and hydrogen atoms omitted for clarity

the molecular structure reveals an eight-coordinate uranium in a distorted dodecahedral configuration. The flexible tridentate [SPSMe ]– anion is bound to the metal as a tertiary phosphine with electronic delocalization within the

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unsaturated parts of the ligand [99]. The U–S distances vary from 2.7799(10) ˚. The two U–P distances ˚ with an average value of 2.88(8) A to 2.9892(12) A Me ˚ are 2.9508(11) and 3.0001(12) A. The SPS ligand can adopt two geometric forms based on the resonance structures presented, where A adopts a facial coordination mode and B is coordinated in a planar fashion. The uranium derivative coordinates one of each type of ligand.

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For ligand B, the relatively large P–C bonds in the central ring show that the ligand lost the ylidic structure of the phosphinine upon coordination to the uranium, while the relatively shorter external P–C bonds (P to C in ring) and the longer P–S bonds are consistent with delocalization of electron density in unsaturated parts of the ligand. The variations observed with the neutral SPS species A result from the presence of the anionic charge.

6 Multimetallic Systems of Uranium Over the last 10–12 years, the growth in uranium coordination complexes has included expansion into the field of multimetallic systems. These complexes offer the potential for a molecule with unique magnetic properties, such as magnetic superexchange, since both a d and f element are held in proximity by a bridging organic ligand framework which provides the magnetic exchange pathway. One proven way to create these multimetallic molecules is to start with a transition metal (M=Co, Ni, Cu, Zn) salen complex that contains a pendant hydroxyl group in the ortho phenyl position [110–113]. This complex is then treated with one/two equivalents of [U(acac)4 ] in the presence of pyridine to form the desired mixed metal complexes of the form [{LMII (py)2 }UIV ] (L = the hexadentate compartmental ligand N,N ′ -bis(3-hydroxysalicylidene)-

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2,2-dimethyl-1,3-propanediamine) with pyridine also coordinated. Characterization of the red cobalt derivative by X-ray crystallography revealed a dodecahedral uranium center coordinated by eight oxygen atoms (Fig. 37). The nickel and zinc compounds are isostructural. The cobalt ion has a square ˚. pyramidal geometry, and is removed slightly from the N2 O2 plane by 0.38(2) A ◦ The Co–U–Co angle is near linear, at 171.84(2) . This family of compounds is the first reported to contain a linear arrangement of three metal atoms. The copper derivative shows remarkable magnetic properties, in that the orbitals of the uranium(IV) center are able to mediate ferromagnetic coupling between the copper atoms. The d(x2 – y2 ) orbitals of the copper ions are coupled through the f (x(y2 – z2 )) and f (y(x2 – z2 )) of the uranium center. Two possibilities for this triplet state are the presence of two degenerate molecular orbitals which contain two unpaired electrons from copper, or the transfer of an unpaired CuII electron toward an empty 5f orbital of uranium, forcing the d and f electrons to align in a parallel arrangement. The magnetic susceptibility (χM T) of this complex is 1.7 cm3 Kmol–1 between 300 and 100 K, which then decreases to 0.8 cm3 Kmol–1 at 2 K. Analogues of this compound, LCu2 Zr and LCu2 Th, were synthesized, and showed magnetic moments of χM T = 0.77 cm3 Kmol–1 over the temperature range, consistent with two noninteracting copper centers, confirming that this ferromagnetic coupling is due to the presence of the uranium center. Due to the lack of coupling of a similar Th derivative, the initial hypothesis about coupling through an empty f or-

Fig. 37 Molecular structure of [LCoII (py)2 UIV ]. Hydrogen atoms omitted for clarity

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bital is disproved. Thus, the conclusion is drawn that the observed coupling is between the 3d unpaired Cu electron and the U 5f electrons. At low T, the uranium(IV) becomes diamagnetic, so the copper species are magnetically isolated from one another. The nickel analogue, Ni2 U, displays antiferromagnetic coupling, with χM T = 2 cm3 Kmol–1 , consistent with two Ni2+ centers. Subsequent to these findings, the diimino chain length and functionality were varied to determine how the coupling of the copper centers is affected by changing the distance between the copper and uranium atoms. For complexes with short Cu–U distances, antiferromagnetic coupling is reported. However, the authors state that ferromagnetic coupling is observed in those complexes that have a long Cu–U interaction, and no interaction between the copper centers is noted [113]. The polydentate monoanionic [Zr2 (Oi Pr)9 ]– (dzni) produces arenesoluble, mixed-metal Zr/U complexes achieved by treating K[Zr2 (Oi Pr)9 ] with [UI3 (THF)4 ], forming [Zr2 (Oi Pr)9 ][UI2 (THF)] in high yields (Fig. 38) [114]. The 1 H NMR spectrum in C6 D6 displays five chemical shifts in a 2 : 2 : 2 : 2 : 1 ratio assigned as the methyl groups of the isopropoxide ligands. Only four of the five expected methane resonances were visible. No fluxionality in this molecule was detected, even at 110 ◦ C. The cyclic voltammogram of [Zr2 (Oi Pr)9 ][UI2 (THF)] recorded in THF displays an irreversible oxidation wave at –0.8 V. Electronic absorption spectroscopy of this complex was performed, and showed strong absorption bands at 436, 501, 612, and 644 nm (ε = 600–900 M–1 cm–1 ). The NIR region appeared similar to

Fig. 38 Molecular structure of [Zr2 (Oi Pr)9 ][UI2 (THF)]. Hydrogen atoms omitted for clarity

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[UI3 (THF)4 ], showing Laporte-forbidden f –f transitions. The integrity of the [{Zr2 (Oi Pr)9 }U]2+ unit was examined by the reaction with K2 C8 H8 which produces the organometallic complex [Zr2 (Oi Pr)9 ][U(C8 H8 )]. 1 H NMR again showed the expected pattern for the methyl groups of the isopropoxide ligands, as well as a resonace at δ –39.8 ppm for the C8 H8 2– ligand. This molecule is fluxional on the NMR timescale. The two isopropyl signals in the 1 H NMR spectrum coalesce at 55 ◦ C. In addition, the 1 H NMR shifts are temperature dependent, and linear with respect to T–1 . Analysis by electrochemistry revealed an irreversible oxidation at –1.5 V. Absorption bands were observed at 447 nm (ε = 950 M–1 cm–1 ) and between 540 and 830 nm (ε = 300–420 M–1 cm–1 ). The NIR region had bands that were more intense and shifted compared to the starting complex. Both of these complexes contain the [Zr2 (Oi Pr)9 ]– unit, which coordinates to U(III) as a tetradentate ligand via two triply bridging and two double bridging isopropoxide oxygen atoms. The reaction of K[Zr2 (Oi Pr)9 ] with UCl4 did not form U(IV)-dzni complexes, and only the ligand exchange product, [UCl2 (Oi Pr)2 (dme)]2 was isolated [114]. The first tris(1,1′ -ferrocenylene) metal compound and the sole homoleptic metal-bridged [1]ferrocenophane was recently reported and crystallographically characterized [115]. Reaction of UCl4 with Li2 fc · tmeda (fc = 1,1′ -ferrocenylene, tmeda = tetramethylethylenediamine) gave the tris(1,1′ ferrocenylene) uranium complex [Li2 (py)3 U(fc)3 ] (py = pyridine) (Fig. 39). The 1 H NMR spectrum has two signals of equal intensity at δ –10.8 and –28.5 assigned as the equivalent protons at the α and β positions of the cyclopentadienyl rings; the most shifted resonance corresponds to the α protons due to the proximity to the paramagnetic uranium center. X-ray analysis

Fig. 39 Molecular structure of [Li2 (py)3 U(fc)3 ]. Hydrogen atoms omitted for clarity

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revealed the propeller type structure of the [U(fc)3 ] fragment, with three ferrocenyl units around the uranium center. The planar cyclopentadienyl rings of each fc group are parallel and with U–C and U–Fe bond distances of 2.52(7) ˚, respectively [115]. The lack of strain between the fc units is reand 3.14(2) A sponsible for the stability of this molecule. The distances between the U and ˚), ˚ (with an average of 3.14(2) A Fe atoms range from 3.122(2) to 3.165(2) A ˚ consistent with the sum of the atomic radii of 3.15 A. There are direct U–Fe and U–Li interactions. Oxidation of a uranium(IV) bis(1,1′ -diamidoferrocene) complex produces a mixed-valent bisferrocene complex in which uranium mediates the electronic communication [116]. Treating [UI3 (THF)4 ] with [K2 (OEt2 )2 ]fc[NSi-(tBu)Me2 ]2 in diethyl ether or toluene led to [U(fc[NSi(t-Bu)Me2 ]2 )2 ]. A similar trimethylsilyl derivative, [U(fc[NSiMe3 ]2 )2 ] was also synthesized (Fig. 40). The cyclic voltammogram (CV) of the free tert-butyl ligand shows one reversible redox process, at –0.60 V vs Cp2 Fe+/0 , consistent with the oxidation of Fe(II) to Fe(III). The corresponding uranium compound shows an irreversible reduction, one quasireversible, and two reversible redox processes at –3.26 (ligand-based reduction), –2.54 (U(IV)/U(III) reduction), –0.69 (Fe(II)/Fe(III) oxidation), and 0.56 V (Fe(II)/Fe(III) oxidation) vs FeCp2 +/0 , respectively. These data support electronic communication between the two iron centers. The low room temperature magnetic moment of 2.50 µB is attributed to an iron–uranium interaction due to orbital overlap. The NIR spectrum of this

Fig. 40 Molecular structure of [U(fc[NSiMe3 ]2 )2 ]. Hydrogen atoms omitted for clarity

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compound displays weak bands consistent with f –f transitions. Chemical oxidation of this compound produced [U(fc[NSi(t-Bu)Me2 ]2 )2 ][BPh4 ], which reportedly has a similar molecular structure to the starting material. Variable temperature magnetization studies produced a magnetic moment of 2.70 µB at 4 K, which increases to 3.01 µB at 40 K and back down to 2.61 µB at room temperature. The low magnetic moments for the uranium compound are partly due to quenching of the orbital angular momentum. Analysis by EPR spectroscopy reveals that the electron is delocalized over the iron centers. Electronic absorption spectroscopy reveals NIR bands with ε ∼ 103 M–1 cm–1 , consistent with an intervalence charge-transfer transition. The IR spectrum shows two bands indicating that both ferrocene and ferrocenium centers are present [116].

7 Recent Highlights and Perspectives Because of its large size and accessibility to multiple oxidation states, uranium is capable of unprecedented reactivity and beautiful coordination complexes that cannot be achieved with transition metals or lanthanides. The exciting products highlighted here demonstrate that we have only just begun to learn the capabilities of uranium, and that continuous studies will be needed to determine the full realm of possibilities. From activation of small molecules to unique magnetic properties, uranium offers a synthetic and spectroscopic challenge to coordination chemists of the future. The first actinide dinitrogen complex was reported in 1998 [117]. Exposure of a benzene-d6 solution of the trivalent complex [U(NN′3 )] to 1 atm of dinitrogen produced the C3 symmetric product [(U(NN′ 3 ))2 {µ2 ,η2 ,η2 -N2 }]. Analysis of the dark red crystals by X-ray diffraction revealed a side-on bridging mode with trigonal monopyramidal uranium centers that are situated out of the planes of the three ligand amido nitrogen atoms by approximately 0.84 ˚ A ˚ is essentially that of free dinitro(Fig. 41). The N–N bond length of 1.109(7) A gen (1.0975 ˚ A) indicating no activation. The 14 N2 and 15 N2 isotopomers have superimposable IR spectra, and the UV/visible electronic absorption spectrum has intense broad bands typical of trivalent uranium complexes [10]. The solution magnetic moment is 3.22 µB (Evans) per uranium atom between 218 and 293 K. It is believed that the preference for side-on over end-on bonding is due to the dinitrogen πp orbital, which is a better σ donor than the σp to trivalent uranium. This dinitrogen binding is reversible as the dinitrogen fragment can be removed during freeze–thaw degassing. Soon after this initial report, a heteronuclear U–Mo dinitrogen compound was reported by Cummins [30]. A 1 : 1 mixture of [(N[R]Ar)3 U(THF)] (R=t–Bu) and [Mo(N[t–Bu]Ph)3 ] in toluene under N2 (1 atm) afforded the end-on orange U(µ2 ,η1 ,η1 -N2 )Mo complex. A proposed hypothesis for the observed result is that the putative dinitrogen complex [(N[t-

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Fig. 41 Molecular structure of [(U(NN′ 3 ))2 {µ2 ,η2 ,η2 -N2 }]. Hydrogen atoms omitted for clarity

Bu]Ph)3 Mo(N2 )] is more efficiently trapped by [(N[R]Ar)3 U(THF)] than by another equivalent of [Mo(N[t-Bu]Ph)3 ]. Inspection by infrared spectroscopy did not produce an obvious stretch for the dinitrogen ligand. However, synthesis with 15 N2 revealed a ν(NN) stretch at 1547 cm–1 . The lack of a band for the 14 N2 isotopomer was attributed to overlap with prominent amide aryl ring ν(CC) stretching modes. Crystallographic analysis revealed an N–N dis˚ (Fig. 42). The shorter distance between the dinitrogen tance of 1.232(11) A ˚, is indicative of some degree of fragment and the uranium center, 2.220(9) A multiple bonding between the two, since the three U–N(amide) distances are A. longer, averaging 2.254 ˚ Gambarotta et al. demonstrated dinitrogen cleavage by treating the starting complex [(Et8 -calix[4]tetrapyrrole)U(dme)][K(dme)] (dme = 1,2dimethoxyethane) with an equivalent of potassium naphthalenide in dme [118]. The result is a mixed-valent µ-nitrido UV/IV complex, which is the product of full dinitrogen cleavage (Fig. 43). The formulation of a pentavalent uranium center was supported by the near-IR spectrum, which displays the characteristic absorption at 1247 nm [119], and supports the formulation of two chemically distinct metal centers. Repeating the reaction in the presence of 15 N2 forms the isotopically labeled complex, which has a distinct hyperfine split EPR spectrum (14 lines) from the 14 N congener. The complex is paramagnetic with a magnetic moment of 3.41 µB (23 ◦ C) per dimeric unit, which is lower than expected. The value of the magnetic moment drops with tem-

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Fig. 42 Molecular structure of [(N[t-Bu]Ar)3 U{µ2 ,η1 ,η1 -N2 }Mo(N[t-Bu]Ph)3 ]. Hydrogen atoms omitted for clarity

Fig. 43 Molecular structure of [(Et8 -calix[4]tetrapyrrole)U(dme)]2(µ-N2 )[K(dme)]. Hydrogen and selected carbon atoms omitted for clarity

perature to 1.91 µB at 2.5 K, with a flex around 10 K that could indicate the presence of substantial antiferromagnetic coupling or superexchange. A previously discussed uranium pentalene complex, [(η5 -Cp∗ )(η8 -C8 H4 i (Si Pr3 -1,4)2 )U], activates dinitrogen to form the side-on coordinated dinitrogen compound [(η5 -Cp∗ )(η8 -C8 H4 (Sii Pr3 -1,4)2 )U]2 {µ2 ,η2 ,η2 -N2 }. Crystallography of the green-black crystals reveals an N1–N2 bond length of ˚, consistent with an N–N double bond (Fig. 44) [120]. In the for1.232(10) A

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Fig. 44 Molecular structure of [(η5 -Cp∗ )(η8 -C8 H4 (Sii Pr3 -1,4)2 )U]2 {µ2 ,η2 ,η2 -N2 }. Hydrogen atoms omitted for clarity

mation of this molecule, two uranium(III) centers each donate an electron to reduce the dinitrogen ligand. Interestingly, this dinitrogen coordination is reversible, as N2 is readily lost in both solution and the solid state. Dinitrogen’s isoelectronic counterpart, carbon monoxide, is a polar molecule, and its coordination and activation should be much more facile. However, activation of carbon monoxide has rarely been studied with uranium coordination complexes. Recently, the first example of a CO-bridged diuranium complex was reported. Addition of CO to a pentane solution of [((tBu ArO)3 tacn)U] to CO (1 atm) produced a gradual color change from redbrown to light brown [121]. Evaporation of the solvent and recrystallization from benzene afforded brown hexagonal crystals of the diuranium species [(((tBu ArO)3 tacn)U)2 {µ2 ,η1 ,η1 -CO}] (Fig. 45). Infrared spectroscopy reveals a band at 2092 cm–1 (Nujol), suggesting a two-coordinate CO molecule. X-ray diffraction revealed the bridging end-on (µ2 : η1 , η1 -CO) coordination mode of CO between the uranium centers. The molecule was modeled as an unsymmetrical U–CO–U entity, with one short U–C bond and a longer U–O isocarbonyl interaction, disordered on two positions at the inversion center. The structure is of limited resolution, resulting in unreliable bond distances for the bridging CO ligand. The average U–O(ArO) and U–N(tacn) distances ˚, respectively, typical for this were determined to be 2.185(5) and 2.676(4) A ligand system. Based on structural parameters, the complex is assigned as a mixed-valent U(III/IV) species, with an average oxidation state of +3.5. The

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Fig. 45 Molecular structure of [(((tBu ArO)3 tacn)U)2 {µ2 ,η1 ,η1 -CO}]. Hydrogen atoms omitted for clarity

formation of this compound is believed to occur via nucleophilic attack of a charge-separated [((tBu ArO)3 tacn)U(IV)-CO• – ] fragment on the coordinatively unsaturated U(III) of [((tBu ArO)3 tacn)U]. Addition of carbon dioxide to the same U(III) starting material produces an interesting reaction as well [121]. In this case, the tert-butyl derivatized aryloxide functionalized triazacyclononane uranium(III) complex, [((tBu ArO)3 tacn)U], performs a one-electron reduction of carbon dioxide to release carbon monoxide and produce a bridging µ-oxo species, [((tBu ArO)3 tacn)U]2 (µ-O). Additionally, the release of carbon monoxide was confirmed crystallographically, as this small molecule is trapped by the highly reactive uranium(III) starting material to yield the previously mentioned bridging end-on [(((tBuArO)3 tacn)U)2 {µ2 ,η1 ,η1 -CO}]. Substituting the ortho tert-butyl substituent on the aryloxide ring for a more sterically bulky adamantyl group changes the reactivity drastically. Using the more bulky ligand set, the U(III) starting material [((Ad ArO)3 tacn)U] was synthesized. Addition of even small amounts of carbon dioxide gas allowed the synthesis and isolation of a uranium–carbon dioxide complex [28]. This compound features an unprecedented η1 -O bound, linear CO2 ligand. Not only is this the first example of a uranium coordination compound with a linear CO2 ligand, but it is the first crystallographic evidence for coordination of carbon dioxide to any metal in this way. The

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carbon dioxide complex [((Ad ArO)3 tacn)U(η1 -OCO)] had a vibrational band at 2188 cm–1 in the infrared spectrum, indicative of a coordinated and activated CO2 ligand. Isotopically labeled 13 CO2 gas produced a shift in the band to 2128 cm–1 . Crystallographic analysis (Fig. 46) of colorless crystals confirmed the linear end-on coordination, and revealed a U–O bond length ˚ and the ter˚. The neighboring C–O bond length is 1.122(4) A of 2.351(3) A ˚. Both the U–O–C and O–C–O angles minal C–O bond length is 1.277(4) A (171.1(2)◦ and 178.0(3)◦ , respectively) are close to linear. The solid-state magnetic moment of the uranium CO2 complex is 2.89 µB at 300 K, and slowly decreases with decreasing temperatures to 2.6 µB at 100 K. Below 100 K, µeff decreases rapidly, reaching a value of 1.51 µB at 5 K. A closed shell U(IV) compound would have a low T moment of approximately 0.5 µB. However, the increased moment of this complex at low temperature supports the formulation of the CO2 fragment as an open shell radical anion. Electronic absorption spectroscopy of this compound revealed weak absorption bands over the entire visible and NIR region assigned to f –f transitions. Taken together, the crystallographic and spectroscopic analyses of this complex are consistent with a formulation of [((Ad ArO)3 tacn)U(η1 -OCO)] as a charge separated uranium(IV) species, [U+ –L· – ], with a radical anion centered on the carbon dioxide ligand [28]. The ability to form a stable charge separated species is unique to uranium, and indicates that this behavior may be important for stabilizing reactive intermediates in uranium-mediated reactions. Uranium complexes have also shown unprecedented reactivity with carbon monoxide. Addition of carbon monoxide to [(COT)(Cp∗ )U(THF)] [122] produced a dimeric C3 O3 2– deltate uranium complex, [{(η5 -Cp∗ )(η8 -COT)U}2

Fig. 46 Molecular structure of [((Ad ArO)3 tacn)U](η1 -OCO). Hydrogen atoms omitted for clarity

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(µ2 ,η1 ,η2 -C3 O3 )], made up of two uranium(IV) centers and a planar core. This compound was characterized by X-ray diffraction to confirm the core, and A above the deltate plane, shows that one uranium atom is displaced by 0.0906 ˚ A below (Fig. 47). The C–O bond distances in the core while the other is 0.1747 ˚ are intermediate of single and double bonds, and there are two short C–C distances and one longer one. The longer bond interacts with the uranium via an agostic interaction, which is confirmed by DFT calculations. This interaction rapidly interconverts between the uranium centers on the NMR timescale, causing a C3 symmetric spectrum at RT. Computation supports that each U is best described as having two electrons localized in 5f orbitals, consistent with the U(IV) formulation. The ancillary COT and Cp ligands bind to the U centers as predicted, with a π interaction between U and Cp and a δ interaction between U and COT. Decorating the COT ring with two i Pr3 Si groups and substituting Cp∗ with CpMe4 provided access to the corresponding red dimeric squarate derivative [29]. This molecule is similar in that the squarate anion is suspended between two U(IV) centers; however, in this case the stabilizing agostic interaction is absent. This is presumably due to the smaller O–C–C angle, which induces bonding to the uranium through only the oxygen atoms. Again, the organic core is planar, but this time the deviation of the uranium centers is much more pronounced; they are situated above and below the core by 0.429 ˚ A. Because the 5f orbitals of actinide ions are more diffuse than the 4f orbitals of the lanthanides, actinides have the potential for stronger magnetic coupling via superexchange [123]. Recently, magnetic exchange in uranium complexes has been studied by synthesis of a series of mixed metal halide-bridged 5f –3d cluster complexes of the form [(cyclam)M[(µCl)U(Me2 Pz)4 ]2 ] (M=Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclotetradecane) (Fig. 48). These are the first examples of halide-bridged species involving uranium(IV) and transition metal ions [124]. The central transition metal has

Fig. 47 Molecular structure of [{(η5 -Cp∗ )(η8 -COT)U}2 (µ2 ,η1 ,η2 -C3 O3 )]. Hydrogen atoms omitted for clarity

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Fig. 48 Molecular structure of [(cyclam)Ni[(µ-Cl)U(Me2 Pz)4 ]2 ]. Hydrogen atoms omitted for clarity

a linear coordination geometry, and forms a chloride-bridged cluster with the uranium centers. The U–Cl–M angle and U(IV) coordination environment vary little as M changes. The species containing the diamagnetic Zn(II) ion is used as a model to account for the U(IV) contributions to the magnetism of the other clusters. Subtracting the Zn data from those obtained for the copper cluster, [(cyclam)Cu[(µ-Cl)U(Me2 Pz)4 ]2 ], reveals a copper center with no magnetic exchange coupling and a slightly lower than expected magnetic moment (1.70(4) µB). The nickel dimer, however, does show magnetic data consistent with the presence of ferromagnetic exchange interactions, indicated by a dip in magnetic moment at 30 K (1.26 emuK/mol). This is most likely due to the loss of U(IV) spin, but probably also from a zero-field splitting contribution to the ground state. The data above 40 K were then fitted and produced a TIP = 8.25 × 10–4 emu/mol. This represents the first estimate of a 5f –3d coupling constant within a molecular complex. The spincontaining orbitals were determined by DFT, and found to be 5f (xyz) and 5f (z(x2 – y2 )), which both exhibit δ symmetry with respect to the U–Cl bond. These are orthogonal to the Ni(II) 3d(z2 ) spin feeding through σ-type Cl orbitals. Thus, the observed ferromagnetic coupling is consistent with a simple superexchange mechanism [124]. The uranyl ion is the longest known and most thoroughly studied uranium complex. Despite this fact, imido analogues of the uranyl derivative have remained elusive until recently, as the trans configuration of the imido ligands is disfavored. In 2005, the first imido derivative was synthesized by addition of tert-butylamine and iodine to uranium turnings in the presence of THF, creating the orange trans-diimide complex with two cis-THFs

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and two cis-iodide ligands (Fig. 49) [125]. Crystallographic analysis reveals A. A similar complex is created by the short U=N distances averaging 1.84 ˚ addition of aniline to [UI3 (THF)4 ]. However, in this case, the resulting uranium(VI) complex has one additional THF ligand, which is coordinated in between the two cis-iodide ligands. This complex has similar crystallographic A. The THF molecules parameters, with an average U=N distance of 1.85 ˚ in both bis-imido derivatives undergo ligand exchange with neutral donors such as aniline to form [U(Nt Bu)2 I2 (NH2 Ph)2 ]. However, trace amounts of water to this complex resulted in the mixed uranium oxo-imido complex ˚ [U(Nt Bu)(O)I2 (THF)(NH2 Ph)2 ] [126]. The U–O bond length of 1.781(4) A is comparable to those found in the uranyl ion, and the U=N(imido) bond ˚ is similar to that of the bis-imido derivatives. The similar length of 1.823(4) A compound [U(Nt Bu)(O)I2 (THF)2 ] was prepared by addition of an equivalent of B(C6 F5 )∗3 H2 O to [U(Nt Bu)2 I2 (THF)2 ]. Infrared spectroscopy of this compound reveals a U–O stretch at 883 cm–1 , which shifts to 827 cm–1 for the 18 O isotopologue. This product undergoes ligand substitution with tppo to form [U(Nt Bu)(O)I2 (Ph3 PO)2 ]. X-ray crystallography of this complex shows similar bond distances to the aniline derivative. The infrared spectrum of a KBr pellet of the tppo derivative exhibits a band at 858 cm–1 (U–O), as well as bands at 1128 and 1134 cm–1 (U–N vibrations and the assignments were confirmed by calculation). DFT geometry optimizations of both the aniline and tppo derivatives show two π-bonding orbitals involved in the U–O bond which have a larger component of d character, while in the U–N bond the uranium f orbitals play a larger role. Because the symmetry of the mixed oxo-imido is less than the bis(imido) system, these M–L multiple bonding interactions have been demonstrated to be more ionic in nature.

Fig. 49 Molecular structure of [U(Nt Bu)2 I2 (THF)2 ]. Hydrogen atoms omitted for clarity

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Fig. 50 Molecular structure of [(Bu4 N)3 ][U(N3 )7 ]. Cation omitted for clarity

Recently, a homoleptic uranium azide anion, UN21 3– , was reported [127]. Addition of Bu4 NBr and seven equivalents of AgN3 to [(Bu4 N)2 ][UCl6 ] in acetonitrile resulted in a color change from pale green to emerald green. Stirring for 12 h followed by isolation produced dark green crystals of the product, [(Bu4 N)3 ][U(N3 )7 ] (Fig. 50). Analysis by X-ray crystallography showed a seven-coordinate uranium center in a monocapped octahedral geometry. ˚. A better quality strucThe U–N bond lengths range from 2.32(2) to 2.40(2) A ture was obtained by changing the reaction and crystallization solvents from CH3 CN to CH3 CH2 CN. This structure showed the uranium center has a pentagonal bipyramidal geometry, with U–N bond lengths ranging from 2.323(6) ˚, ˚. The Nα –Nβ bond lengths range from 1.162(8) to 1.246(9) A to 2.431(7) A and are longer than the corresponding Nβ –Nγ bond lengths of 1.055(8)– ˚. The angles within the five equatorial azide ligands deviate from 1.150(7) A linearity, with angles ranging from 164(1) to 168.1(8)◦ , while those in the apical positions have angles of 179.5(7) and 178.9(9)◦ [127].

8 Closing Remarks The work presented here demonstrates the great variety of ligands and coordination modes that have already been studied with uranium. Despite that fact, there is so much still to be learned about the ability of uranium to

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coordinate organic ligands, activate small molecules, and perform catalytic chemistry. This review presents just a few examples of the unprecedented chemistry already being studied. The size and highly reducing nature of uranium compared to transition metals promise that this actinide will play an important role in making an impact on modern society.

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Author Index Volumes 101–130 Author Index Vols. 1–100 see Vol. 100

The volume numbers are printed in italics Alajarin M, see Turner DR (2004) 108: 97–168 Aldinger F, see Seifert HJ (2002) 101: 1–58 Aldridge S, see Kays DL (2008) 130: 29–122 Alessio E, see Iengo E (2006) 121: 105–143 Alfredsson M, see Corà F (2004) 113: 171–232 Aliev AE, Harris KDM (2004) Probing Hydrogen Bonding in Solids Using State NMR Spectroscopy 108: 1–54 Alloul H, see Brouet V (2004) 109: 165–199 Amstutz N, see Hauser A (2003) 106: 81–96 Anitha S, Rao KSJ (2003) The Complexity of Aluminium-DNA Interactions: Relevance to Alzheimer’s and Other Neurological Diseases 104: 79–98 Anthon C, Bendix J, Schäffer CE (2004) Elucidation of Ligand-Field Theory. Reformulation and Revival by Density Functional Theory 107: 207–302 Aramburu JA, see Moreno M (2003) 106: 127–152 Arˇcon D, Blinc R (2004) The Jahn-Teller Effect and Fullerene Ferromagnets 109: 231–276 Arman HD, see Pennington WT (2007) 126: 65–104 Aromí G, Brechin EK (2006) Synthesis of 3d Metallic Single-Molecule Magnets. 122: 1–67 Atanasov M, Daul CA, Rauzy C (2003) A DFT Based Ligand Field Theory 106: 97–125 Atanasov M, see Reinen D (2004) 107: 159–178 Atwood DA, see Conley B (2003) 104: 181–193 Atwood DA, Hutchison AR, Zhang Y (2003) Compounds Containing Five-Coordinate Group 13 Elements 105: 167–201 Atwood DA, Zaman MK (2006) Mercury Removal from Water 120: 163–182 Autschbach J (2004) The Calculation of NMR Parameters in Transition Metal Complexes 112: 1–48 Baerends EJ, see Rosa A (2004) 112: 49–116 Balch AL (2007) Remarkable Luminescence Behaviors and Structural Variations of TwoCoordinate Gold(I) Complexes. 123: 1–40 Bara JE, see Gin DL (2008) 128: 181–222 Baranoff E, Barigelletti F, Bonnet S, Collin J-P, Flamigni L, Mobian P, Sauvage J-P (2007) From Photoinduced Charge Separation to Light-Driven Molecular Machines. 123: 41–78 Barbara B, see Curély J (2006) 122: 207–250 Bard AJ, Ding Z, Myung N (2005) Electrochemistry and Electrogenerated Chemiluminescence of Semiconductor Nanocrystals in Solutions and in Films 118: 1–57 Barigelletti F, see Baranoff E (2007) 123: 41–78 Barriuso MT, see Moreno M (2003) 106: 127–152

178

Author Index Volumes 101–130

Bart SC, Meyer K (2008) Highlights in Uranium Coordination Chemistry. 127: 119–176 Beaulac R, see Nolet MC (2004) 107: 145–158 Bebout DC, Berry SM (2006) Probing Mercury Complex Speciation with Multinuclear NMR 120: 81–105 Bellamy AJ (2007) FOX-7 (1,1-Diamino-2,2-dinitroethene). 125: 1–33 Bellandi F, see Contreras RR (2003) 106: 71–79 Bendix J, see Anthon C (2004) 107: 207–302 Berend K, van der Voet GB, de Wolff FA (2003) Acute Aluminium Intoxication 104: 1–58 Berry SM, see Bebout DC (2006) 120: 81–105 Besora M, Lledós A (2008) Coordination Modes and Hydride Exchange Dynamics in Transition Metal Tetrahydroborate Complexes. 130: 149–202 Bianconi A, Saini NL (2005) Nanoscale Lattice Fluctuations in Cuprates and Manganites 114: 287–330 Biella S, see Metrangolo P (2007) 126: 105–136 Blinc R, see Arcˇcon D (2004) 109: 231–276 Blinc R (2007) Order and Disorder in Perovskites and Relaxor Ferroelectrics. 124: 51–67 Boˇca R (2005) Magnetic Parameters and Magnetic Functions in Mononuclear Complexes Beyond the Spin-Hamiltonian Formalism 117: 1–268 Bohrer D, see Schetinger MRC (2003) 104: 99–138 Bonnet S, see Baranoff E (2007) 123: 41–78 Bouamaied I, Coskun T, Stulz E (2006) Axial Coordination to Metalloporphyrins Leading to Multinuclear Assemblies 121: 1–47 Boulanger AM, see Nolet MC (2004) 107: 145–158 Boulon G (2004) Optical Transitions of Trivalent Neodymium and Chromium Centres in LiNbO3 Crystal Host Material 107: 1–25 Bowlby BE, Di Bartolo B (2003) Spectroscopy of Trivalent Praseodymium in Barium Yttrium Fluoride 106: 193–208 Braga D, Maini L, Polito M, Grepioni F (2004) Hydrogen Bonding Interactions Between Ions: A Powerful Tool in Molecular Crystal Engineering 111: 1–32 Braunschweig H, Kollann C, Seeler F (2008) Transition Metal Borylene Complexes. 130: 1–27 Brechin EK, see Aromí G (2006) 122: 1–67 Brouet V, Alloul H, Gàràj S, Forrò L (2004) NMR Studies of Insulating, Metallic, and Superconducting Fullerides: Importance of Correlations and Jahn-Teller Distortions 109: 165– 199 Bruce DW (2007) Halogen-bonded Liquid Crystals. 126: 161–180 Buddhudu S, see Morita M (2004) 107: 115–144 Budzelaar PHM, Talarico G (2003) Insertion and β-Hydrogen Transfer at Aluminium 105: 141–165 Burrows AD (2004) Crystal Engineering Using Multiple Hydrogen Bonds 108: 55–96 Bussmann-Holder A, Dalal NS (2007) Order/Disorder Versus or with Displacive Dynamics in Ferroelectric Systems. 124: 1–21 Bussmann-Holder A, Keller H, Müller KA (2005) Evidences for Polaron Formation in Cuprates 114: 367–386 Bussmann-Holder A, see Dalal NS (2007) 124: 23–50 Bussmann-Holder A, see Micnas R (2005) 114: 13–69 Byrd EFC, see Rice BM (2007) 125: 153–194 Canadell E, see Sánchez-Portal D (2004) 113: 103–170 Cancines P, see Contreras RR (2003) 106: 71–79 Caneschi A, see Cornia A (2006) 122: 133–161

Author Index Volumes 101–130

179

Cartwright HM (2004) An Introduction to Evolutionary Computation and Evolutionary Algorithms 110: 1–32 Chapman RD (2007) Organic Difluoramine Derivatives. 125: 123–151 Christie RA, Jordan KD (2005) n-Body Decomposition Approach to the Calculation of Interaction Energies of Water Clusters 116: 27–41 Clérac R, see Coulon C (2006) 122: 163–206 Cloke FGN, see Summerscales OT (2008) 127: 87–117 Clot E, Eisenstein O (2004) Agostic Interactions from a Computational Perspective: One Name, Many Interpretations 113: 1–36 Collin J-P, see Baranoff E (2007) 123: 41–78 Conley B, Atwood DA (2003) Fluoroaluminate Chemistry 104: 181–193 Contakes SM, Nguyen YHL, Gray HB, Glazer EC, Hays A-M, Goodin DB (2007) Conjugates of Heme-Thiolate Enzymes with Photoactive Metal-Diimine Wires. 123: 177–203 Contreras RR, Suárez T, Reyes M, Bellandi F, Cancines P, Moreno J, Shahgholi M, Di Bilio AJ, Gray HB, Fontal B (2003) Electronic Structures and Reduction Potentials of Cu(II) Complexes of [N,N′ -Alkyl-bis(ethyl-2-amino-1-cyclopentenecarbothioate)] (Alkyl = Ethyl, Propyl, and Butyl) 106: 71–79 Cooke Andrews J (2006) Mercury Speciation in the Environment Using X-ray Absorption Spectroscopy 120: 1–35 Corà F, Alfredsson M, Mallia G, Middlemiss DS, Mackrodt WC, Dovesi R, Orlando R (2004) The Performance of Hybrid Density Functionals in Solid State Chemistry 113: 171–232 Cornia A, Costantino AF, Zobbi L, Caneschi A, Gatteschi D, Mannini M, Sessoli R (2006) Preparation of Novel Materials Using SMMs. 122: 133–161 Coskun T, see Bouamaied I (2006) 121: 1–47 Costantino AF, see Cornia A (2006) 122: 133–161 Coulon C, Miyasaka H, Clérac R (2006) Single-Chain Magnets: Theoretical Approach and Experimental Systems. 122: 163–206 Crespi VH, see Gunnarson O (2005) 114: 71–101 Curély J, Barbara B (2006) General Theory of Superexchange in Molecules. 122: 207–250 Dalal NS, Gunaydin-Sen O, Bussmann-Holder A (2007) Experimental Evidence for the Coexistence of Order/Disorder and Displacive Behavior of Hydrogen-Bonded Ferroelectrics and Antiferroelectrics. 124: 23–50 Dalal NS, see Bussmann-Holder A (2007) 124: 1–21 Daul CA, see Atanasov M (2003) 106: 97–125 Day P (2003) Whereof Man Cannot Speak: Some Scientific Vocabulary of Michael Faraday and Klixbüll Jørgensen 106: 7–18 Deeth RJ (2004) Computational Bioinorganic Chemistry 113: 37–69 Delahaye S, see Hauser A (2003) 106: 81–96 Deng S, Simon A, Köhler J (2005) Pairing Mechanisms Viewed from Physics and Chemistry 114: 103–141 Di Bartolo B, see Bowlby BE (2003) 106: 191–208 Di Bilio AJ, see Contreras RR (2003) 106: 71–79 Ding Z, see Bard AJ (2005) 118: 1–57 Dovesi R, see Corà F (2004) 113: 171–232 Duan X, see He J (2005) 119: 89–119 Duan X, see Li F (2005) 119: 193–223 Egami T (2005) Electron-Phonon Coupling in High-Tc Superconductors 114: 267–286 Egami T (2007) Local Structure and Dynamics of Ferroelectric Solids. 124: 69–88

180

Author Index Volumes 101–130

Eisen MS, see Sharma M (2008) 127: 1–85 Eisenstein O, see Clot E (2004) 113: 1–36 Ercolani G (2006) Thermodynamics of Metal-Mediated Assemblies of Porphyrins 121: 167– 215 Evans DG, see He J (2005) 119: 89–119 Evans DG, Slade RCT (2005) Structural Aspects of Layered Double Hydroxides 119: 1–87 Ewing GE (2005) H2 O on NaCl: From Single Molecule, to Clusters, to Monolayer, to Thin Film, to Deliquescence 116: 1–25 Flamigni L, Heitz V, Sauvage J-P (2006) Porphyrin Rotaxanes and Catenanes: Copper(I)Templated Synthesis and Photoinduced Processes 121: 217–261 Flamigni L, see Baranoff E (2007) 123: 41–78 Fontal B, see Contreras RR (2003) 106: 71–79 Forrò L, see Brouet V (2004) 109: 165–199 Fourmigué M (2007) Halogen Bonding in Conducting or Magnetic Molecular Materials. 126: 181–207 Fowler PW, see Soncini A (2005) 115: 57–79 Frenking G, see Lein M (2003) 106: 181–191 Frühauf S, see Roewer G (2002) 101: 59–136 Frunzke J, see Lein M (2003) 106: 181–191 Funahashi M, Shimura H, Yoshio M, Kato T (2008) Functional Liquid-Crystalline Polymers for Ionic and Electronic Conduction. 128: 151–179 Furrer A (2005) Neutron Scattering Investigations of Charge Inhomogeneities and the Pseudogap State in High-Temperature Superconductors 114: 171–204 Gao H, see Singh RP (2007) 125: 35–83 Gàràj S, see Brouet V (2004) 109: 165–199 Gatteschi D, see Cornia A (2006) 122: 133–161 Gillet VJ (2004) Applications of Evolutionary Computation in Drug Design 110: 133–152 Gin DL, Pecinovsky CS, Bara JE, Kerr RL (2008) Functional Lyotropic Liquid Crystal Materials. 128: 181–222 Glazer EC, see Contakes SM (2007) 123: 177–203 Golden MS, Pichler T, Rudolf P (2004) Charge Transfer and Bonding in Endohedral Fullerenes from High-Energy Spectroscopy 109: 201–229 Goodby JW, see Saez IM (2008) 128: 1–62 Goodin DB, see Contakes SM (2007) 123: 177–203 Gorelesky SI, Lever ABP (2004) 107: 77–114 Grant GJ (2006) Mercury(II) Complexes with Thiacrowns and Related Macrocyclic Ligands 120: 107–141 Grätzel M, see Nazeeruddin MK (2007) 123: 113–175 Gray HB, see Contreras RR (2003) 106: 71–79 Gray HB, see Contakes SM (2007) 123: 177–203 Grepioni F, see Braga D (2004) 111: 1–32 Gritsenko O, see Rosa A (2004) 112: 49–116 Güdel HU, see Wenger OS (2003) 106: 59–70 Gunnarsson O, Han JE, Koch E, Crespi VH (2005) Superconductivity in Alkali-Doped Fullerides 114: 71–101 Gunter MJ (2006) Multiporphyrin Arrays Assembled Through Hydrogen Bonding 121: 263– 295 Gunaydin-Sen O, see Dalal NS (2007) 124: 23–50

Author Index Volumes 101–130

181

Gütlich P, van Koningsbruggen PJ, Renz F (2004) Recent Advances in Spin Crossover Research 107: 27–76 Guyot-Sionnest P (2005) Intraband Spectroscopy and Semiconductor Nanocrystals 118: 59–77 Habershon S, see Harris KDM (2004) 110: 55–94 Han JE, see Gunnarson O (2005) 114: 71–101 Hanks TW, see Pennington WT (2007) 126: 65–104 Hardie MJ (2004) Hydrogen Bonded Network Structures Constructed from Molecular Hosts 111: 139–174 Harris KDM, see Aliev (2004) 108: 1–54 Harris KDM, Johnston RL, Habershon S (2004) Application of Evolutionary Computation in Structure Determination from Diffraction Data 110: 55–94 Hartke B (2004) Application of Evolutionary Algorithms to Global Cluster Geometry Optimization 110: 33–53 Harvey JN (2004) DFT Computation of Relative Spin-State Energetics of Transition Metal Compounds 112: 151–183 Haubner R, Wilhelm M, Weissenbacher R, Lux B (2002) Boron Nitrides – Properties, Synthesis and Applications 102: 1–46 Hauser A, Amstutz N, Delahaye S, Sadki A, Schenker S, Sieber R, Zerara M (2003) Fine Tuning the Electronic Properties of [M(bpy)3 ]2+ Complexes by Chemical Pressure (M = Fe2+ , Ru2+ , Co2+ , bpy = 2,2′ -Bipyridine) 106: 81–96 Hays A-M, see Contakes SM (2007) 123: 177–203 He J, Wei M, Li B, Kang Y, G Evans D, Duan X (2005) Preparation of Layered Double Hydroxides 119: 89–119 Heitz V, see Flamigni L (2006) 121: 217–261 Herrmann M, see Petzow G (2002) 102: 47–166 Herzog U, see Roewer G (2002) 101: 59–136 Hoggard PE (2003) Angular Overlap Model Parameters 106: 37–57 Höpfl H (2002) Structure and Bonding in Boron Containing Macrocycles and Cages 103: 1–56 Hubberstey P, Suksangpanya U (2004) Hydrogen-Bonded Supramolecular Chain and Sheet Formation by Coordinated Guranidine Derivatives 111: 33–83 Hupp JT (2006) Rhenium-Linked Multiporphyrin Assemblies: Synthesis and Properties 121: 145–165 Hutchison AR, see Atwood DA (2003) 105: 167–201 Iengo E, Scandola F, Alessio E (2006) Metal-Mediated Multi-Porphyrin Discrete Assemblies and Their Photoinduced Properties 121: 105–143 Itoh M, Taniguchi H (2007) Ferroelectricity of SrTiO3 Induced by Oxygen Isotope Exchange. 124: 89–118 Iwasa Y, see Margadonna S (2004) 109: 127–164 Jansen M, Jäschke B, Jäschke T (2002) Amorphous Multinary Ceramics in the Si-B-N-C System 101: 137–192 Jäschke B, see Jansen M (2002) 101: 137–192 Jäschke T, see Jansen M (2002) 101: 137–192 Jaworska M, Macyk W, Stasicka Z (2003) Structure, Spectroscopy and Photochemistry of the [M(η5 -C5 H5 )(CO)2 ]2 Complexes (M = Fe, Ru) 106: 153–172 Jenneskens LW, see Soncini A (2005) 115: 57–79

182

Author Index Volumes 101–130

Jeziorski B, see Szalewicz K (2005) 116: 43–117 Johnston RL, see Harris KDM (2004) 110: 55–94 Jordan KD, see Christie RA (2005) 116: 27–41 Kabanov VV, see Mihailovic D (2005) 114: 331–365 Kang Y, see He J (2005) 119: 89–119 Karpfen A (2007) Theoretical Characterization of the Trends in Halogen Bonding. 126: 1–15 Kato T, see Funahashi M (2008) 128: 151–179 Kays DL, Aldridge S (2008) Transition Metal Boryl Complexes. 130: 29–122 Keller H (2005) Unconventional Isotope Effects in Cuprate Superconductors 114: 143–169 Keller H, see Bussmann-Holder A (2005) 114: 367–386 Kerr RL, see Gin DL (2008) 128: 181–222 Khan AI, see Williams GR (2005) 119: 161–192 Kikuchi H (2008) Liquid Crystalline Blue Phases. 128: 99–117 Kind R (2007) Evidence for Ferroelectric Nucleation Centres in the Pseudo-spin Glass System Rb1-x (ND4 )x D2 PO4 : A 87 Rb NMR Study. 124: 119–147 Klapötke TM (2007) New Nitrogen-Rich High Explosives. 125: 85–121 Kobuke Y (2006) Porphyrin Supramolecules by Self-Complementary Coordination 121: 49–104 Koch E, see Gunnarson O (2005) 114: 71–101 Kochelaev BI, Teitel’baum GB (2005) Nanoscale Properties of Superconducting Cuprates Probed by the Electron Paramagnetic Resonance 114: 205–266 Kochi JK, see Rosokha SV (2007) 126: 137–160 Köhler J, see Deng (2005) 114: 103–141 Kollann C, see Braunschweig H (2008) 130: 1–27 van Koningsbruggen, see Gütlich P (2004) 107: 27–76 Kume S, Nishihara H (2007) Metal-Based Photoswitches Derived from Photoisomerization. 123: 79–112 Lee M, see Ryu J-H (2008) 128: 63–98 Legon AC (2007) The Interaction of Dihalogens and Hydrogen Halides with Lewis Bases in the Gas Phase: An Experimental Comparison of the Halogen Bond and the Hydrogen Bond. 126: 17–64 Lein M, Frunzke J, Frenking G (2003) Christian Klixbüll Jørgensen and the Nature of the Chemical Bond in HArF 106: 181–191 Leroux F, see Taviot-Gueho C (2005) 119: 121–159 Lever ABP, Gorelesky SI (2004) Ruthenium Complexes of Non-Innocent Ligands; Aspects of Charge Transfer Spectroscopy 107: 77–114 Li B, see He J (2005) 119: 89–119 Li F, Duan X (2005) Applications of Layered Double Hydroxides 119: 193–223 Liebau F, see Santamaría-Pérez D (2005) 118: 79–135 Linton DJ, Wheatley AEH (2003) The Synthesis and Structural Properties of Aluminium Oxide, Hydroxide and Organooxide Compounds 105: 67–139 Lin Z (2008) Transition Metal σ -Borane Complexes. 130: 123–148 Lledós A, see Besora M (2008) 130: 149–202 Lo KK-W (2007) Luminescent Transition Metal Complexes as Biological Labels and Probes. 123: 205–245 Lux B, see Haubner R (2002) 102: 1–46

Author Index Volumes 101–130

183

Mackrodt WC, see Corà F (2004) 113: 171–232 Macyk W, see Jaworska M (2003) 106: 153–172 Mahalakshmi L, Stalke D (2002) The R2M+ Group 13 Organometallic Fragment Chelated by P-centered Ligands 103: 85–116 Maini L, see Braga D (2004) 111: 1–32 Mallah T, see Rebilly J-N (2006) 122: 103–131 Mallia G, see Corà F (2004) 113: 171–232 Mannini M, see Cornia A (2006) 122: 133–161 Margadonna S, Iwasa Y, Takenobu T, Prassides K (2004) Structural and Electronic Properties of Selected Fulleride Salts 109: 127–164 Maseras F, see Ujaque G (2004) 112: 117–149 Mather PT, see Rowan SJ (2008) 128: 119–149 Mattson WD, see Rice BM (2007) 125: 153–194 McInnes EJL (2006) Spectroscopy of Single-Molecule Magnets. 122: 69–102 Merunka D, Rakvin B (2007) Anharmonic and Quantum Effects in KDP-Type Ferroelectrics: Modified Strong Dipole–Proton Coupling Model. 124: 149–198 Meshri DT, see Singh RP (2007) 125: 35–83 Metrangolo P, Resnati G, Pilati T, Biella S (2007) Halogen Bonding in Crystal Engineering. 126: 105–136 Meyer K, see Bart SC (2008) 127: 119–176 Micnas R, Robaszkiewicz S, Bussmann-Holder A (2005) Two-Component Scenarios for Non-Conventional (Exotic) Superconstructors 114: 13–69 Middlemiss DS, see Corà F (2004) 113: 171–232 Mihailovic D, Kabanov VV (2005) Dynamic Inhomogeneity, Pairing and Superconductivity in Cuprates 114: 331–365 Millot C (2005) Molecular Dynamics Simulations and Intermolecular Forces 115: 125–148 Miyake T, see Saito (2004) 109: 41–57 Miyasaka H, see Coulon C (2006) 122: 163–206 Mobian P, see Baranoff E (2007) 123: 41–78 Moreno J, see Contreras RR (2003) 106: 71–79 Moreno M, Aramburu JA, Barriuso MT (2003) Electronic Properties and Bonding in Transition Metal Complexes: Influence of Pressure 106: 127–152 Morita M, Buddhudu S, Rau D, Murakami S (2004) Photoluminescence and Excitation Energy Transfer of Rare Earth Ions in Nanoporous Xerogel and Sol-Gel SiO2 Glasses 107: 115–143 Morsch VM, see Schetinger MRC (2003) 104: 99–138 Mossin S, Weihe H (2003) Average One-Center Two-Electron Exchange Integrals and Exchange Interactions 106: 173–180 Murakami S, see Morita M (2004) 107: 115–144 Müller E, see Roewer G (2002) 101: 59–136 Müller KA (2005) Essential Heterogeneities in Hole-Doped Cuprate Superconductors 114: 1–11 Müller KA, see Bussmann-Holder A (2005) 114: 367–386 Myung N, see Bard AJ (2005) 118: 1–57 Nazeeruddin MK, Grätzel M (2007) Transition Metal Complexes for Photovoltaic and Light Emitting Applications. 123: 113–175 Nguyen YHL, see Contakes SM (2007) 123: 177–203 Nishibori E, see Takata M (2004) 109: 59–84 Nishihara H, see Kume S (2007) 123: 79–112

184

Author Index Volumes 101–130

Nolet MC, Beaulac R, Boulanger AM, Reber C (2004) Allowed and Forbidden d-d Bands in Octa-hedral Coordination Compounds: Intensity Borrowing and Interference Dips in Absorption Spectra 107: 145–158 O’Hare D, see Williams GR (2005) 119: 161–192 Ordejón P, see Sánchez-Portal D (2004) 113: 103–170 Orlando R, see Corà F (2004) 113: 171–232 Oshiro S (2003) A New Effect of Aluminium on Iron Metabolism in Mammalian Cells 104: 59–78 Pastor A, see Turner DR (2004) 108: 97–168 Patkowski K, see Szalewicz K (2005) 116: 43–117 Patoˇcka J, see Strunecká A (2003) 104: 139–180 Pecinovsky CS, see Gin DL (2008) 128: 181–222 Peng X, Thessing J (2005) Controlled Synthesis of High Quality Semiconductor Nanocrystals 118: 137–177 Pennington WT, Hanks TW, Arman HD (2007) Halogen Bonding with Dihalogens and Interhalogens. 126: 65–104 Petzow G, Hermann M (2002) Silicon Nitride Ceramics 102: 47–166 Pichler T, see Golden MS (2004) 109: 201–229 Pilati T, see Metrangolo P (2007) 126: 105–136 Polito M, see Braga D (2004) 111: 1–32 Popelier PLA (2005) Quantum Chemical Topology: on Bonds and Potentials 115: 1–56 Power P (2002) Multiple Bonding Between Heavier Group 13 Elements 103: 57–84 Prassides K, see Margadonna S (2004) 109: 127–164 Prato M, see Tagmatarchis N (2004) 109: 1–39 Price LS, see Price SSL (2005) 115: 81–123 Price SSL, Price LS (2005) Modelling Intermolecular Forces for Organic Crystal Structure Prediction 115: 81–123 Rabinovich D (2006) Poly(mercaptoimidazolyl)borate Complexes of Cadmium and Mercury 120: 143–162 Rakvin B, see Merunka D (2007) 124: 149–198 Rao KSJ, see Anitha S (2003) 104: 79–98 Rau D, see Morita M (2004) 107: 115–144 Rauzy C, see Atanasov (2003) 106: 97–125 Reber C, see Nolet MC (2004) 107: 145–158 Rebilly J-N, Mallah T (2006) Synthesis of Single-Molecule Magnets Using Metallocyanates. 122: 103–131 Reinen D, Atanasov M (2004) The Angular Overlap Model and Vibronic Coupling in Treating s-p and d-s Mixing – a DFT Study 107: 159–178 Reisfeld R (2003) Rare Earth Ions: Their Spectroscopy of Cryptates and Related Complexes in Glasses 106: 209–237 Renz F, see Gütlich P (2004) 107: 27–76 Resnati G, see Metrangolo P (2007) 126: 105–136 Reyes M, see Contreras RR (2003) 106: 71–79 Ricciardi G, see Rosa A (2004) 112: 49–116 Rice BM, Byrd EFC, Mattson WD (2007) Computational Aspects of Nitrogen-Rich HEDMs. 125: 153–194

Author Index Volumes 101–130

185

Riesen H (2004) Progress in Hole-Burning Spectroscopy of Coordination Compounds 107: 179–205 Robaszkiewicz S, see Micnas R (2005) 114: 13–69 Roewer G, Herzog U, Trommer K, Müller E, Frühauf S (2002) Silicon Carbide – A Survey of Synthetic Approaches, Properties and Applications 101: 59–136 Rosa A, Ricciardi G, Gritsenko O, Baerends EJ (2004) Excitation Energies of Metal Complexes with Time-dependent Density Functional Theory 112: 49–116 Rosokha SV, Kochi JK (2007) X-ray Structures and Electronic Spectra of the π-Halogen Complexes between Halogen Donors and Acceptors with π-Receptors. 126: 137–160 Rowan SJ, Mather PT (2008) Supramolecular Interactions in the Formation of Thermotropic Liquid Crystalline Polymers. 128: 119–149 Rudolf P, see Golden MS (2004) 109: 201–229 Ruiz E (2004) Theoretical Study of the Exchange Coupling in Large Polynuclear Transition Metal Complexes Using DFT Methods 113: 71–102 Ryu J-H, Lee M (2008) Liquid Crystalline Assembly of Rod–Coil Molecules. 128: 63–98 Sadki A, see Hauser A (2003) 106: 81–96 Saez IM, Goodby JW (2008) Supermolecular Liquid Crystals. 128: 1–62 Saini NL, see Bianconi A (2005) 114: 287–330 Saito S, Umemoto K, Miyake T (2004) Electronic Structure and Energetics of Fullerites, Fullerides, and Fullerene Polymers 109: 41–57 Sakata M, see Takata M (2004) 109: 59–84 Sánchez-Portal D, Ordejón P, Canadell E (2004) Computing the Properties of Materials from First Principles with SIESTA 113: 103–170 Santamaría-Pérez D, Vegas A, Liebau F (2005) The Zintl–Klemm Concept Applied to Cations in Oxides II. The Structures of Silicates 118: 79–135 Sauvage J-P, see Flamigni L (2006) 121: 217–261 Sauvage J-P, see Baranoff E (2007) 123: 41–78 Scandola F, see Iengo E (2006) 121: 105–143 Schäffer CE (2003) Axel Christian Klixbüll Jørgensen (1931–2001) 106: 1–5 Schäffer CE, see Anthon C (2004) 107: 207–301 Schenker S, see Hauser A (2003) 106: 81–96 Schetinger MRC, Morsch VM, Bohrer D (2003) Aluminium: Interaction with Nucleotides and Nucleotidases and Analytical Aspects of Determination 104: 99–138 Schmidtke HH (2003) The Variation of Slater-Condon Parameters Fk and Racah Parameters B and C with Chemical Bonding in Transition Group Complexes 106: 19–35 Schubert DM (2003) Borates in Industrial Use 105: 1–40 Schulz S (2002) Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi 103: 117–166 Scott JF (2007) A Comparison of Magnetic Random Access Memories (MRAMs) and Ferroelectric Random Access Memories (FRAMs). 124: 199–207 Seeler F, see Braunschweig H (2008) 130: 1–27 Seifert HJ, Aldinger F (2002) Phase Equilibria in the Si-B-C-N System 101: 1–58 Sessoli R, see Cornia A (2006) 122: 133–161 Shahgholi M, see Contreras RR (2003) 106: 71–79 Sharma M, Eisen MS (2008) Metallocene Organoactinide Complexes. 127: 1–85 Shimura H, see Funahashi M (2008) 128: 151–179 Shinohara H, see Takata M (2004) 109: 59–84 Shreeve JM, see Singh RP (2007) 125: 35–83 Sieber R, see Hauser A (2003) 106: 81–96

186

Author Index Volumes 101–130

Simon A, see Deng (2005) 114: 103–141 Singh RP, Gao H, Meshri DT, Shreeve JM (2007) Nitrogen-Rich Heterocycles. 125: 35–83 Slade RCT, see Evans DG (2005) 119: 1–87 Soncini A, Fowler PW, Jenneskens LW (2005) Angular Momentum and Spectral Decomposition of Ring Currents: Aromaticity and the Annulene Model 115: 57–79 Stalke D, see Mahalakshmi L (2002) 103: 85–116 Stasicka Z, see Jaworska M (2003) 106: 153–172 Steed JW, see Turner DR (2004) 108: 97–168 Strunecká A, Patoˇcka J (2003) Aluminofluoride Complexes in the Etiology of Alzheimer’s Disease 104: 139–180 Stulz E, see Bouamaied I (2006) 121: 1–47 Suárez T, see Contreras RR (2003) 106: 71–79 Suksangpanya U, see Hubberstey (2004) 111: 33–83 Summerscales OT, Cloke FGN (2008) Activation of Small Molecules by U(III) Cyclooctatetraene and Pentalene Complexes. 127: 87–117 Sundqvist B (2004) Polymeric Fullerene Phases Formed Under Pressure 109: 85–126 Szalewicz K, Patkowski K, Jeziorski B (2005) Intermolecular Interactions via Perturbation Theory: From Diatoms to Biomolecules 116: 43–117 Tagmatarchis N, Prato M (2004) Organofullerene Materials 109: 1–39 Takata M, Nishibori E, Sakata M, Shinohara M (2004) Charge Density Level Structures of Endohedral Metallofullerenes by MEM/Rietveld Method 109: 59–84 Takenobu T, see Margadonna S (2004) 109: 127–164 Talarico G, see Budzelaar PHM (2003) 105: 141–165 Taniguchi H, see Itoh M (2007) 124: 89–118 Taviot-Gueho C, Leroux F (2005) In situ Polymerization and Intercalation of Polymers in Layered Double Hydroxides 119: 121–159 Teitel’baum GB, see Kochelaev BI (2005) 114: 205–266 Thessing J, see Peng X (2005) 118: 137–177 Trommer K, see Roewer G (2002) 101: 59–136 Tsuzuki S (2005) Interactions with Aromatic Rings 115: 149–193 Turner DR, Pastor A, Alajarin M, Steed JW (2004) Molecular Containers: Design Approaches and Applications 108: 97–168 Uhl W (2003) Aluminium and Gallium Hydrazides 105: 41–66 Ujaque G, Maseras F (2004) Applications of Hybrid DFT/Molecular Mechanics to Homogeneous Catalysis 112: 117–149 Umemoto K, see Saito S (2004) 109: 41–57 Unger R (2004) The Genetic Algorithm Approach to Protein Structure Prediction 110: 153–175 van der Voet GB, see Berend K (2003) 104: 1–58 Vegas A, see Santamaría-Pérez D (2005) 118: 79–135 Vilar R (2004) Hydrogen-Bonding Templated Assemblies 111: 85–137 Wei M, see He J (2005) 119: 89–119 Weihe H, see Mossin S (2003) 106: 173–180 Weissenbacher R, see Haubner R (2002) 102: 1–46 Wenger OS, Güdel HU (2003) Influence of Crystal Field Parameters on Near-Infrared to Visible Photon Upconversion in Ti2+ and Ni2+ Doped Halide Lattices 106: 59–70

Author Index Volumes 101–130

187

Wheatley AEH, see Linton DJ (2003) 105: 67–139 Wilhelm M, see Haubner R (2002) 102: 1–46 Williams GR, Khan AI, O’Hare D (2005) Mechanistic and Kinetic Studies of Guest Ion Intercalation into Layered Double Hydroxides Using Time-resolved, In-situ X-ray Powder Diffraction 119: 161–192 de Wolff FA, see Berend K (2003) 104: 1–58 Woodley SM (2004) Prediction of Crystal Structures Using Evolutionary Algorithms and Related Techniques 110: 95–132 Xantheas SS (2005) Interaction Potentials for Water from Accurate Cluster Calculations 116: 119–148 Yoshio M, see Funahashi M (2008) 128: 151–179 Zaman MK, see Atwood DA (2006) 120: 163–182 Zeman S (2007) Sensitivities of High Energy Compounds. 125: 195–271 Zerara M, see Hauser A (2003) 106: 81–96 Zhang H (2006) Photochemical Redox Reactions of Mercury 120: 37–79 Zhang Y, see Atwood DA (2003) 105: 167–201 Zobbi L, see Cornia A (2006) 122: 133–161

Subject Index

Actinide, homoleptic 156 Actinide butadiene 69 Actinide carbonyl 99 Actinide complexes, synthesis/reactivity 4 –, trivalent Cp 4 Actinide diene, ring inversion 70 Actinide dinitrogen 163 Activation 87 An(IV) tris cyclopentedienyl 53 Arene ligands 4 Azobenzene 12 Bis(cyclopentadienyl)actinides 71 Bis(cyclopentadienyl)thorium dialkyl 72 1,2-Bis(dimethylphosphino)ethane 19 2,6-Bis(imidazolylidene)pyridine 144 Bis(pentalene) U(IV) 90 Bis(pentamethylcyclopentadienyl) derivatives, thorium/uranium 53 Bis(pentamethylcyclopentadienyl) metallocene dithiolates 68 Bis(pentamethylcyclopentadienyl)thorium dialkyl 72 Bis(pentamethylcyclopentadienyl)uranium 32, 64 Bis(substituted cyclopentadinyl)actinide(IV) 56 Bis(trimethylsilyl)squarate 107 Bis-arylactinide 72 Bridging complexes 20

Chalcogen bridged uranium(IV) complexes 23 Chalcogen-containing ligands 149 3-Chloropyridine 27 Coordination chemistry 119 Copper 159 Croconate 102 Cyanoimides 140 Cyclohexadienyl 4 Cyclooctatetraene (COT) 87 –, U(III), mixed sandwich 89 Cyclooctatetraenyl 4 Cyclopentadienyl 4 –, permethylated 8 Cyclopentadienyl thallium 36 Cyclopentadienyl U(III) isocyanide complexes 31 Cyclopentadienyl/dithiolene 33 Deltate 103 –, formation 99 5,6-Dihydro-1,4-dithiin-2,3-dithiolate 33 1,3-Dimesitylimidazole-2-ylidene 136 4-Dimethylaminopyridine 19 3,5-Dimethylpyrazine 27 Dinitrogen 96 Diorganophosphido actinide 68 Dithiolene 149 Electronic spectroscopy Ethyne diolate 104 Fischer–Tropsch

Carbamoyl methyl sulfoxide ligands, bifunctional 135 Carbocycles, silylated eight-membered 91 Carbon dioxide 108, 167 Carbon monoxide 98, 166

95

106

Halide bridge U(III) complexes 21 Hexakisamido uranium 140 Hydrides ligands 4 Hydroamination/cyclization 122 Hydrocarbyls 4

190

Subject Index

Indenyl 4 Isocyanate actinide 133 Isocyanates 112 Isocyanides adducts, uranium(III) metallocenes 31

Squarate 103 –, formation 104 Steric coordination number 4 Sterically induced reduction (SIR) Sulfur ligands 135

K/18-crown-6/benzene

Tetrahydrofuran (THF) adducts 5 Tetrahydrothiophene 19 Tetrakis(cyclopentadienyl) ligand 35 Tetrakis(dithiolene) 151 Tetramethylfulvene 9 Tetravalent chemistry 35 Thorium 2 Thorium butadiene 69 Thorium hydrocarbyls, CO insertion 48 Thorium(IV) tetraazamacrocycle 61 Transition metal–phosphine imine 52 s-Triazine 27 Tri(cyclopentadienyl)AnX 38 Trimethylphosphine 19 Tris(N-tert-butylanilide) 145 Tris-Cp complexes 32 Tris(cyclopentadienyl)actinides 71 Tris(cyclopentadienyl)diethylaminouranium 41 Tris(cyclopentadienyl)thorium 8 Tris(1,1′ -ferrocenylene) 161 Tris(indenyl)thorium 37 Tris(indenyl)uranium 37 Tris(pyrazol-1-yl)hydroborate 124 Trithiacrown 130 1,4,7-Trithiacyclononane 130

14

Lanthanides 35 Lewis bases, affinity 25 Ligand cone angles 3 Ligands, multiply bonded 3,5-Lutidine 27

130

Macrocyclic ligands 122 Metal ligand back donation 28 Metallocene complexes 1 Molecular and electronic structure 119 Mono(cyclopentadienyl) uranium(III) 34 Multimetallic systems, uranium 158 Nitrogen donor ligands 140 Organoactinides 1 Organoimido ligands 65 Organouranium(III) amide 18 Oxocarbons, CO 102 –, U(IV)-bound, functionalisation/extraction 106 Pentalene 87 Pentalene complexes 89 Phosphaalkyne 110 Phosphine imides 52 Phospholyl 4 Phosphorous species 110 3-Picoline 27 Plutonium 30 Poly(thioimidazolyl)borate 124 Pyrazine 27 Pyridazine 27 Pyrimidine 27 Reduction chemistry 87 Rhodizonate 102 Salen ligands 138 Small molecules 87 –, activation 96, 119

1, 10

U–CO–U 166 UI3 , solvent-free 121 UI3 (bipy)2 (py) 122 U–Mo dinitrogen 163 Uranium 87, 119 –, multimetallic systems 158 Uranium azide, homoleptic 172 Uranium coordination chemistry 120 Uranium pentalene 165 Uranium pentamethylcyclopentadienyl 17 Uranium phosphorus complexes 49 Uranium precursors 122 Uranium silyl, non-metallocene 145 Uranium thiolate 152 Uranium–nitrogen multiple bond 51 Uranium–pyridyl 124

Subject Index Uranium(III) 122 –, halide bridge 21 Uranium(III) acetonitrile salt 122 Uranium(III) cyclooctatetraene, mixed sandwich 89 Uranium(III) nitrile 112 Uranium(III) pentamethylcyclopentadienyl 9 Uranium(III) thiolato complexes 25 Uranium(IV) 143 –, chalcogen bridged 23 Uranium(IV) bis(1,1‘-diamidoferrocene) 162

191 Uranium(IV) dithiolene 149 Uranium(IV) halide 122 Uranium(IV) organoimido 66 Uranium(IV) phosphoylide 50 Uranium(IV) triflate 122 Uranium(IV)-bound oxocarbons, functionalisation/extraction 106 Uranium(V) imido 142 Uranocene 124 Uranyl ion 130 Zirconocene–dinitrogen Zr/U 160

104